What are the legal implications of all of this?
Person B has a viable claim of defamation for statements falsely attributed to him.
If the false representations are severe by falsely attributing to him felonies or moral turpitude, it is defamation per se and therefore person B is not required to prove damages.
In order to be awarded more than nominal damages of $1 (one dollar) for defamation per se, person B will need to prove that person A made the false statements with actual malice (that is, with reckless disregard of the truth or knowing them to be false), which seems quite doable in the scenario you describe.
For false representations that are less severe than that, person B will have to prove that he incurred losses as a result thereof. In this situation, person B will need to prove actual malice only if person A can reasonably advance the defense of qualified privilege (which is unlikely in the situation you describe) or if person B is a public figure.
Other claims such as tortious interference with relations derive from defamation, and thus they are treated as a claim of defamation.
Would impersonating someone with the purpose of trashing someone's name and promoting it be enough to pursue something more than a civil case?
Impersonation with intent to deceive others as to someone's identity is a crime (regardless of the propensity --in each jurisdiction-- to prosecute it).
That being said, there is an important distinction between "intent or reasonable effect of deceiving" and parody. The latter usually is obvious enough in that it does not attempt to mislead others as to the source or authorship of statements. Was person A's impersonation of B a parody that made truthful but uncomfortable disclosures?
In the case of parody, what matters is whether the import is defamatory. For instance, in the latter portion of this post, I make a parody of judge felon Carol Kuhnke, who last year endorsed a sexual harasser for judicial reelection. The parody simulates as if felon Kuhnke wrote a letter to the assistant who for years was sexually harassed by Kuhnke's judicial buddy.
The parody-letter obviously does not intend to deceive others as to its authorship: you will notice that the line preceding the parody letter presents it as a plausible adaptation of Kuhnke's statements in court. And several items preempt any "defamatory import", since
- the parody mirrors Kuhnke's statements during a court hearing she presided;
- judge Kuhnke was busted for illegal possession of drugs in year 2016;
- Kuhnke endorsed her colleague Gregg Iddings in 2018 despite him having gotten suspended by the Michigan Supreme Court for sexually harassing his secretary; and
- Kuhnke's campaign endorsement of a sexual harasser is quite at odds with Kuhnke's campaign babbling on how "it's important that a woman's voice be heard in all of the matters of society".
Thus, merging all these facts (with their sources) to substantiate the parody simply leaves no room for claims of defamation. Except for the form (letter), the substance is true and therefore does not carry an actionable defamatory import.
Similarly, another parody is a simulation of judge Larsen's letter to three innocent siblings after she favored the judge who in year 2015 were yelled at, extorted, and jailed them for 17 days. Obviously the judge did not write the letter in the post, but the substance conveyed by that parody is premised on verifiable facts.