If I’ve read my dev blogs correctly & haven’t attributed something I read about something else to this then…
iirc one of the new structures will be anchored nearby any contents that can be will be transferred to the new structure & the rest asset safety’d… ownership goes to the owner of the original structure (of course) or in the case of free ports (is it?) is given to the last corp to hold ownership.
I might of course be confusing myself with something else though so could be completely wrong 
No iirc they’re replaced by other structures then go poof, see above.
No, you just keep them, they’ll have no use or value but can probably still be contract sold (some people collect old stuff), there’s never compensation for defunct BPO’s.
EDIT:
Ah… no I don’t think I had, I think that’s all just player owned outposts (whatever the heck they are) & conquerable stations (these I know of), POS may just be going poof & that’s all she wrote?