S.W.
As for what to say in an interview to the question "Why did you leave your last job?" At the outplacement company that I consult with, we call this a "reason for leaving statement".
Write out a statement and practice saying it out loud and changing the wording until it feels right to you. Start with a very short, factual statement about what happened. I agree that you can contact your HR person to find out what information would be shared with a potential employer if they call the company. Base your statement off of that. If they aren't going to state that you were terminated, you can use something generic like "the job became not a good fit for me", but if they are going to reveal that you were terminated, you might want to say "there was a misunderstanding that led to my termination".
Next in your statement shift to a positive statement about what you are proud of having done, something you learned or accomplished, during the time you were there. "I'm glad that while I was at the company, I was able to ....".
Then end with a statement about the future. "And now I'm looking forward to bringing my skills and experience to a new company."
All this should be less than a 30 second answer. Keep it short, simple, factual, unemotional.
So, for example "Over time, due to some changes, the position with company X was no longer a good fit for me. I'm happy that I was able to learn several new skills and complete an important project before I left. Now, I'm excited to find a new company that can benefit from my experience and skills."