The general rule
Is a person in one of those two scenarios forever in purgatory where
theoretically charges are always potentially pending?
In the first scenario, yes.
In the second scenario, "it depends."
In the third scenario, jeopardy does not attach (and hence bar a future prosecution) until a jury is sworn or a witness is called in a bench trial.
Even if charges are brought, double jeopardy doesn't preclude a new trial until one of those two things happen, and even then, a retrial is allowed if there is a mistrial that is not the fault of the prosecution or if there is a conviction that is vacated on appeal for a new trial.
Also, a state court trial doesn't bar a federal prosecution (even if it produces an acquittal) and visa versa.
Options to definitively resolve potential criminal charges
If so, is there some scenario or legal way of definitively closing out
the potential of future charges short of being charged and winning in
Generally, you can't do anything.
Not really, but a few potential exceptions are discussed below.
While it is rare, it isn't unheard of for criminal charges to be brought decades later when an investigation was initially inconclusive. This has mostly happened in U.S. civil rights era cases, in cases arising out Nazi war crimes, and in cold cases where new evidence (often DNA evidence, but sometimes confessions or other evidence).
Civil lawsuits generally aren't an option
A civil action for a declaratory judgment that you were not guilty would usually be dismissed on the grounds that it is "not ripe for adjudication" if no charges are pending (which deprives the court of subject-matter jurisdiction over the case).
Agreements with the DA
The best option that is usually available is to reach an agreement with the District Attorney (DA) (i.e. criminal prosecutor), although this, of course, requires the DA to be willing to do so, which would be uncommon.
A DA can make a binding agreement that the DA's office in question (even with a different DA holding that office) will not prosecute the case in the future. A Nolle prosequi declaration would frequently be issued following such an agreement.
One famous case where such an agreement was ultimately upheld (following a trial ending in a mistrial, a retrial resulting in a conviction, a first appeal upholding the verdict, and an ultimate vindication in the state supreme court) was the prosecution of Bill Cosby for sexual assault. But, as the procedural history of that case illustrates, enforcing such agreements can be difficult due to fights over the scope of the agreement and its applicability to the case in question.
An agreement with the DA isn't perfect protection.
In our federal system, many murders are both state and federal crimes, and an agreement with a state prosecutor wouldn't bar a federal prosecution (and vice versa).
Also, in some states a DA's agreement would only bind that office and not the state attorney general, which often also has authority to prosecute state law crimes that a DA does not prosecute, while in other states, the DA's agreement would bind all state prosecutors since the DA is an agent of the state and "the People" in whose name the charges are brought.
Special state self-defense laws may exist in a handful of states
I believe that there may be a handful of U.S. states that specifically provides for a proceeding by which someone who asserts self-defense can get a binding acquittal, under recent pro-self-defense legislation.
But this is rarely an option, since few jurisdictions allow it and there are often other limitations on seeking this remedy (e.g., it might only be available if there has been an indictment) where it is available.
In Florida, for example, you can have a binding pre-trial determination of a judge that you were exonerated based upon self-defense, but this right is triggered only once you are prosecuted, even though it can apply before jeopardy attaches for double jeopardy purposes.
Pardons are possible in some cases
A pardon would also prevent any risk of prosecution.
But, while uncharged conduct can be pardoned for federal crimes, only a minority of U.S. states allow for the pardon of uncharged conduct. Also, while this is a disputed matter and a pardon doesn't amount to an admission of civil liability, many people interpret acceptance of a pardon as an admission of guilt, which could harm the reputation of someone who asserts and believes that they did not commit a crime.
Statutes of limitations eventually bar most non-murder charges
Of course, in most U.S. states and in the District of Columbia, almost all crimes except murder and some fraud offenses and sometimes certain sex offenses have a statute of limitations that bars prosecutions after a certain period of time (two to seven years after the crime was committed with a handful of exceptions). Most federal crimes have a five year statute of limitations.
Sometimes only true murder has no statute of limitations in states that have criminal statutes of limitations for felonies, but in other states, there is no statute of limitations for any homicide charges or for any homicide charges other than criminally negligent homicide.
Seven states do not have a statute of limitations, for almost any felonies (sometimes with minor exceptions). South Carolina and Wyoming are the only states without a statute of limitations for misdemeanors.
In the case of an arguable murder where self-defense is claimed, statute of limitations won't normally be an issue. But, for the vast majority of criminal charges in most U.S. jurisdictions, the statute of limitations will bar future charges.
The federal law statutes of limitations in criminal cases are summarized here:
What Is The Federal Statute Of Limitations – Generally?
The federal statute of limitations for most offenses is five years
from the time the offense occurred. 18 U.S.C. § 3282.
What Are Federal Crimes That Have No Statute Of Limitations?
There are some serious crimes that do not carry a statute of
limitations and can be prosecuted at any time:
Any crime where death could be punishment.18 U.S.C. § 3281.
Crimes involving terrorism that results in serious bodily injury or
create a risk of death or serious injury. 18 U.S.C. § 3286.
Crimes, including kidnapping, sexual abuse, and sexual exploitation,
where the victim is a minor. 18 U.S.C. § 3299.
What Are Federal Crimes With Longer Statute Of Limitations?
In many other cases, the statute of limitation still exists, but is
longer than the general five-year rule. These include, but are not
Crimes involving terrorism 18 U.S.C. 3286 8 years
Wartime Fraud 18 U.S.C. 3287 5 years from end of war
Child Kidnap, Sexual or Physical Abuse 18 U.S.C. 3283 Longer of 10
years or life of victim
Crimes involving nationality, citizenship, or passports 18 U.S.C.
3291 10 years Crimes against Financial Institutions 18 U.S.C. 3293 10
Theft of major artwork 18 U.S.C. 3294 20 years
Non-capital Arson offenses 18 U.S.C. 3295 10 years
Trafficking offenses 18 U.S.C. 3298 10 years
Recruitment of child soldiers 18 U.S.C. 3300 10 years
Federal tax evasion/fraud against the IRS 26 U.S.C. 6531 6 years
Major Fraud against the US 18 U.S.C. 1031 7 years
Data Offenses under Atomic Energy Laws 42 U.S.C. 2278 10 years
Offenses under Subversive Activities Control Act 50 U.S.C. 783 10
What Are Federal Crimes With Shorter Statute Of Limitations?
Some crimes also have shorter statutes of limitations. These are
somewhat more limited and in specific instances. In tax crimes, most
statutes of limitations are three years instead of five years (not
including those stated in the chart above). 26 U.S.C. § 6531. Criminal
contempt’s statute of limitations is only one year long, if brought
under certain parts of the code. 18 U.S.C § 3285.
There are certain crimes that have special rules regarding the
beginning of the statute of limitations.
The running of limitations typically begin after the last element has
been satisfied. In the case of conspiracy, there will be many overt
acts in the commission of the ultimate offense. The statute of
limitations for conspiracy will begin at the final overt act. Fiswick
v. United States, 329 U.S. 211, 216 (1946). Other criminal acts are
also considered continuing crimes for the purposes of the statute of
limitations including: escape or flight from custody or prosecution,
possession of contraband, kidnapping, concealment of assets of in
bankruptcy, and failure to pay child support.
Statute of limitations can also be delayed from beginning by other
circumstances. In cases where DNA is involved, the statute of
limitations is delayed for the time it takes to use the DNA for
identification. 18 U.S.C. § 3297. The statute of limitations, in that
circumstance, begins to run after the DNA has identified the
wrongdoer. Under another statute, the use of DNA in an aggravated
sexual abuse case to procure an indictment for the DNA profile
suspends the statute of limitations. 18 U.S.C. § 3282. When an
indictment or information is dismissed, the government gets an
additional six months on the statute of limitations. 18 U.S.C. § 3288.
When the information or evidence needed for the prosecution is located
in a foreign area, the court is able to suspend the statute of
limitations to accommodate. 18 U.S.C. § 3292. In addition, any person
fleeing from justice shall not enjoy the benefit of the statute of
limitations. 18 U.S.C. § 3290.
State statutes of limitations in criminal cases are summarized here. States without a statute of limitations for felonies include Kentucky, Maryland, North Carolina, South Carolina, Virginia, West Virginia (except certain kinds of perjury with a 3 year statute of limitations), and Wyoming.
For non-capital felonies and non-fraud offenses, most states have a statutes of limitations of 3-7 years is typical. Pennsylvania and Mississippi have a 2 year statute of limitations for felonies. Tennessee has statutes of limitations from 2-15 years depending upon the offense for these felonies. Many states with a criminal statute of limitations have selected exceptions to the general rule that may be fact specific (e.g., cases with DNA evidence or crimes against children) or exceptions that are charge specific.
Why Are Charges Filed Late?
I believe (please correct me if wrong) that one of the most likely
scenarios that could result in future re-opening the case and/or
charges would be establishing a new "fact pattern". If this is true,
would the person speaking publicly on the issue be enough to
potentially establish this foothold? In other words, let's say the
person does an interview with a news organization. Let's assume they
aren't entirely stupid and say something like, "I wanted to kill the
guy and I took my shot", but instead just talk about the situation and
maybe they describe a scenario which makes the self-defense claim seem
a bit gray. Would that potentially be enough?
In the U.S., no new facts are needed to bring a criminal case that isn't barred by the statute of limitations or double jeopardy. While cold cases often are refiled based upon new evidence, public attention to a case in which there is no new evidence is also often a factor.
Another factor is shifting public opinion. Many civil rights era cases were not brought because anti-civil rights sentiment at the time meant that a jury was unlikely to convict someone for killing a black man or a civil rights worker, with a jury likely to resort to jury nullification even if the evidence of guilt was overwhelming. That sentiment faded over time and a prosecutor might bring a case now because the fear of an unfavorable jury no longer discourages prosecution.
Additional Procedural Consideration
It is common in cases of killings by law enforcement officers and in self-defense cases, where the DA doesn't really believe that a murder was committed but wants to diffuse blame for not bringing charges, for the DA to present the case in a neutral fashion (as opposed to the usual strong one-sided advocacy for an indictment) to a grand jury. In these cases, the grand jury usually, although not always, declines to indict, even though usually 99.9% of cases presented to grand juries result in indictments.
While a "no bill" from a grand jury is not a legal bar to a prosecution, a grand jury finding that the state did not have probable cause to prosecute the case strongly discourages future prosecutors from bringing a case.