1a. The fact that the manager showed up with the "30 day performance review" speech means that for sure they want to fire you, and also that he has already has this signed off by HR. Therefore going to HR, at least about the specific issue of being fired, is not useful.
1b. There is some very small chance that 1a is not true and in fact the manager is a total loose canon, or is bluffing. Up to you to assess which is the case but 1a is much more likely.
2. A trick I've used myself in the many situations I've found myself in over the years is this: imagine they make a movie about what goes down. Imagine you being played by some big name actor in the movie (the movie "Margin Call" might be a good concrete example here). Now: play time forward and consider what the guy playing your character in the movie would say when he testifies in court. Here's the important part: make sure that your actions today are consistent with that guy looking good and ending the movie not in jail :)
Put another way: think about how you would fare if you have to stand up in court at some later date and explain your actions. Think about the emails you send, the letters you write particularly.
In your case, "looking good in the movie" might involve taking the advice other folks have given here: seek legal advice and then follow it.