Don't know, A and C!
It was on the Piper Arrow which I used for my CPL at EFT, and I only logged about 10 hours on it. What I've described is my understanding of the system, as described to me by one of the instructors (not my regular instructor, though).
When I suffered a vacuum failure, I told my instructor pretty much what I've described above. Sure enough, that was exactly how it behaved - I cross-checked with the compass regularly, and found the two to be in agreement the whole time (well - as close to agreement as they ever were before the vacuum failure) except after we'd done some steep turns, when the "Hdg" flag popped up, and it took a minute or two for everything to re-align again.
The instructor (also not my regular instructor, but not the same one who explained the system to me to start with) didn't correct me during the flight (which I wouldn't have expected him to), but also didn't say anything about the HSI/RMI after the flight. I took the fact that he didn't say anything after the flight to mean that his understanding was the same as mine.
Don't know if anyone at EFT reads this forum - if they do maybe they'd be able to find out what type it was and post here? And if my understanding of how it works was wrong, then I'd like to know - just for general interest, since I'll most likely never fly that aircraft again.
FFF
-------------