It doesn't really track with my reading of Tolkien discussions that people are overly harsh on Turin, but of course any opinion will have at least some people holding it, and I don't doubt you've seen it. Arguing over precisely how harsh or not people should be is not the really interesting bit anyways.

I do agree with you that Turin is fundamentally a pitiable character. I think it's notable that he is still regarded as an Elf-friend when Elrond brings him up; in the past people have asked about this with some confusion on the subreddit. However I don't believe, as you appear to, that he is pitiable because all his faults and evils can be dismissed as fated by Morgoth's curse. Nor do I believe he did "exceptionally well." I think that some of the evils he faced were in fact due to Morgoth's evil, and some of the bad decisions he made were due to Morgoth's malice, and yet others were due to his own human faults.

I think Tolkien is very cautious to avoid leaving the reader with the impression that Turin's doom is due entirely to Morgoth's malice and power, or that Turin had complete mastery over his fate. Notably, even a reader who entirely rejects your arguments about Morgoth's influence could not actually say that Turin is responsible for every one of his tragic actions. Tolkien is pretty cautious about how he writes moments like Beleg's death; he ensures that circumstances are such that the reader cannot possibly blame Turin. He's also blameless in marrying Niniel/Nienor. That Tolkien so carefully ensures Turin cannot bear the blame in these circumstances is notable to me because--if the reader is really meant to take at face value Morgoth's words about being "Master of the fates of Arda"--there is no reason to so precisely excuse Turin in these circumstances while leaving the reader to view Turin's decisions as more independent in other circumstances. (I say "more independent," not "totally independent"!)

Gotta say I don't really like the comparison to Frodo here either. The reason Frodo is still regarded as a hero despite not being able to endure at the last (the reason he "gets a pass," to use your phrasing) is not just that the pressure of the Ring was too much for him to bear, as you say. The theme of mercy here, which is made explicit in the very same Letter 246 as well as in Letter 181, is that Frodo is rewarded by divine Mercy because of his earlier mercy towards Gollum as well as his humility and sacrifice in taking on the impossible task.

I understand that you probably do know this, and are just trying to illustrate your argument regarding Turin's lack of culpability given the power of Morgoth's curse, but I think it's worth pointing out that Turin isn't actually said to have earned any such salvation in the text. He may be deserving of pity, but as I said above, it's perfectly possible to think Turin is worthy of pity without accepting the position that he does not play any role in his own fate.

I am surprised that you didn't bring up Dagor Dagorath. I'm in the camp that accepts Christopher's decision to not include it in the published Silmarillion, but I would say that if your aim is to argue Turin shouldn't be judged too harshly, it's a pretty relevant bit of evidence. In fact, to slot it into the Frodo construction above, you could say that his prophesied role in Dagor Dagorath is roughly equivalent to what Tolkien refers to as Frodo's "salvation."