One question at a time, lady!

[He shakes his head, trying to sort out how to answer all that.]

Don't think it's about my perception, but I never tested it. Seems anything lower than room temp does it. Anything as warm as bath water or hotter will change me back.

No idea what happens to my extra mass, it's magic, so maybe it just literally disappears?

No, I can't choose to do it. Which is why I still consider it a curse, even though I consider both bodies as mine now.