- With these facts, assuming Mr Y was charged with involuntary manslaughter (like in MA v. Carter) or aiding a suicide, based solely
or almost solely on the messages, under which jurisdiction would he be
States have jurisdiction both over crimes that are committed in the state and over crimes that cause harm in a state.
The classic law school example is a murder committed by shooting someone with a gun across a state line. Both the state where the gun is fired and the state where the person is shot have jurisdiction over the crime.
Jurisdiction generally requires a purposeful act directed at someone or something in the state where the harm is suffered in most cases. But that isn't a hard and fast rule of constitutional law in other contexts, and there are few cases on point. I would consider this to be an open question.
Certainly, however, the mere fact that the victim of a crime is transported to another state for medical treatment, where that victim then dies from causes relate to the crime, does not give the state where the death ultimately occurs in the hospital jurisdiction over the offense.
Indeed, the constitutional protection against double jeopardy does not prohibit both states from independently convicting and punishing the same defendant for the same crime in this situation under the "dual sovereignty" doctrine. As background, the Colorado Supreme Court decided a dual sovereignty double jeopardy case today.
Application To Facts
(The application to the facts has been revised upon closer examination of them.)
The line about "Ms X, who is, at this point, still in Nevada," is confusing because she was in California before and isn't described as ever being in Nevada. I presume that "still in California" was really meant.
Mr. Y could be charged (at least) in Nevada or New Jersey from which the continuing course of communications was sent (undue emphasis on the final communication is probably inappropriate), and Wyoming, to which the bulk of the communications were directed and where the bulk of the harm was suffered.
California and Nebraska do not seem to be places to which the communications were really directed or where the greatest harm was suffered. Momentary presence in Nebraska air space is probably insufficient.
There are also a set of statutes that specifically address crimes committed during an airplane flight (see also here) that has been discussed in other answers at this website. To the extent that this is treated as a homicide committed while in flight, 49 USC § 46506, might also allow for a federal criminal prosecution.
I'm not sure that this is really a crime committed in flight, however, as it involved a course of conduct. A single email or a single moment of death doesn't really capture it. It is more analogous to a poisoning taking place in many doses over a period of time.
- Is Mr Y's speech in this case protected by the First Amendment?
No. First Amendment considerations do apply to crimes involving communications between people that are not false, but if there is sufficient intent to cause suicide or other harm, the First Amendment yields to other considerations. The freedom of speech is not absolute. The exact place that the line is drawn is a matter of ongoing litigation.
This specific issue is explored in depth in Clay Calvert, "The First Amendment and Speech Urging Suicide: Lessons from the Case of Michelle Carter and the Need to Expand Brandenburg Application" 94 Tulane Law Review 79 (November 2019).
This article is responsive to that case of Commonwealth v. Carter, 115 N.E.3d 559 (Mass. 2019). The article explains in its introduction that:
In February 2019, the Massachusetts Supreme Judicial Court in
Commonwealth v. Carter' affirmed Michelle Carter's conviction for
involuntary manslaughter as a youthful offender based on her urging
Conrad Roy to commit suicide.' In doing so, the court rejected
Carter's claim that her conviction violated her First Amendment' right
of free speech. Specifically, it reasoned that Carter's words with Roy
immediately before and while he died were "integral to a course of
criminal conduct and thus [did] not raise any constitutional problem."
In brief, Massachusetts's high court concluded that Carter's speech
caused Roy's death' and that the First Amendment provided her no