…
…
…
…you got the twist right on your first guess! I won’t question you on the small details; a gentleman knows when he’s been beat. There were a few minor Red you didn’t explain, but an explanation’s easy enough to guess, given the solution you have. Congratulations!
Given that the original story didn’t have things like “personal characters” and “motive,” there’s not much to speculate on why the murders happened. To put it simply, ‘A’ lured them there to kill them. The mangled corpse was actually a disguised killer, and ‘A’ talked to them over the phone to convince them he was there; hence the “faint voice from deep in the suite.” From there, it’s as you said, basically. All the Red Truth was preventing ‘A’ from doing anything, and the initial Red Truth was ambiguous enough to support, but not confirm, the assumption that it was ‘A’ in the master suite.