@TomD8 @Opus 17
Someone who lives in the house and assumes the "burdens of ownership" would probably be considered an equitable owner or beneficial owner, even if he is not the legal owner. An equitable owner can deduct mortgage interest and real estate tax if he paid them.
26 CFR 1.163-1(b)
" Interest paid by the taxpayer on a mortgage upon real estate of which he is the legal or equitable owner, . . ."
There is also case law on this.
As I said in my first reply above, "It's complicated."
(Thanks to DoninGA for pointing out the CFR reference a few years ago.)
