If I can find pro bono legal help like Morgana suggests,
These would probably be just enough to get you started doing some research, rather than full-on legal help.
But you got me wondering whether your age is high enough or your income low enough to qualify for actual full-on legal help. Might be worth looking into.
finding another person who has been through the process would be like gold.
Agree 100%.
Unfortunately, I’m in cognitive decline and not quite as resourceful as I used to be.
Well, based on your posts in general, I'd say you're doing a lot with what you've got. And this is a challenge that would stump anyone.
Meanwhile, back to the top of the thread,
what’s required for a property to be considered a residence.
You might want to consider making a list to answer the question "considered a residence for what purposes?" IIRC this started about driver's licenses, but you also mentioned mail delivery; then there's insurance, voter registration, taxation ... it might be that there's a different law governing each, rather than one law governing "what counts as a residence." (Hope not! but could be.)
If you had a list handy, that might make it easier for whoever's giving you legal aid/research pointers to give you a good thorough answer.
PS
Other thought:
The mailbox one might be big (since so many things depend on being able to get mail). And that might be a good thread to start tugging on, if you can't get more comprehensive advice elsewhere. ("Hello, Post Office, what do I have to do to get mail delivery at xyz location?" "You have to get abcd from the County." "Hello, County, what do I have to do to get abcd?" "You have to ..." etc.) I mean, give even the most unhelpful bureaucrat the chance to say "No
[I can't help you until you do X]" or "You must
[do X]" and it'll be like catnip.
Good luck!