fair warning this is mostly speculation because it's only a single book/6 ep TV series so the world building isn't really deep.
1. He definitely isn't possessing someone; angels/demons refer to their bodies as 'corporations', the general assumption is that they're made by Heaven/Hell respectively (they can't seem to do it themselves) and issued to them the way someone would be given a work uniform. It appears human, aside from his eyes & wings, but as mentioned, doesn't need maintenance and is difficult to kill (in canon if that happened, he'd pop back to Hell, fill out some paperwork and get a new body). Sometimes he turns into a snake but who knows what's up there and is something that would presumably be nerfed.
2. Welp, that'll suck for him, but ty!
3. I'd think they're more magical than physical! But I could work with them being there, just inaccessible.
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1. He definitely isn't possessing someone; angels/demons refer to their bodies as 'corporations', the general assumption is that they're made by Heaven/Hell respectively (they can't seem to do it themselves) and issued to them the way someone would be given a work uniform. It appears human, aside from his eyes & wings, but as mentioned, doesn't need maintenance and is difficult to kill (in canon if that happened, he'd pop back to Hell, fill out some paperwork and get a new body). Sometimes he turns into a snake but who knows what's up there and is something that would presumably be nerfed.
2. Welp, that'll suck for him, but ty!
3. I'd think they're more magical than physical! But I could work with them being there, just inaccessible.