Is anyone dealing with an elderly parent who has basically disowned them?

Yes. Different circumstances. Same result. No solutions for you, sorry. I feel stress every minute of every day because of this. I pray that for you and me, time will heal the relationships and the wounds. I try to maintain my normal life and enjoy my friends and family. It's tough. We cannot let it affect our families. This is how I deal with this:
-- Do something to relieve the stress. I hang with friends who understand and also go to the gym. The physical exercise provides a great outlet.
-- Don't blame her for her actions and words. She may have dementia or other form of cognitive impairment.
-- Identify when your mother was still her old self. That's your mom. She is still in there. Love and care for that person.

When you say "has always been a little prickly with people" do you mean she has always been a raging misanthropist? Ascribing the worst possible motives to others' actions, with negative expectations of everyone she comes into contact with? Does she have, has she ever had, a spontaneous good word to say about anybody?

What cahoots did she suspect you of being in with the neighbour?

I haven't experienced this myself, however have heard this played out a few times now.

Sometimes it's part of tuning out socially overall. Paranoid thinking causing distrust of everyone.

Sometimes blame aimed only at next of kin or POA. Sometimes revoking POA. If that person was the eldest adult child, they may move down the list to the next suitable adult child. Once children are exhausted, nieces & nephews start getting calls. The kindest or most sympathetic sounding becomes the new favourite - for a while.

Arranging 'other eyes' to check in can be beneficial if the main person is no longer able to. Eg using Condo Manager, other relatives or Social Welfare Agencies.

My MIL has disowned my DH because he is married to me.

Yet he still is her executor. Not looking forward to THAT hot mess when she passes.

We did hear this kind of second hand, and his feelings are that if his mom is trying to get him to divorce me over his inheritance, she can go ahead, but he sure doesn't want to do all the crappy work that an executor does.

"will use me when she is ready. "

Just make sure she doesn't use you to be her maid or caregiving slave.

I doubt your mother despises you, in the traditional sense of the word. She's likely suffering from dementia with what you describe about 'paranoia' worsening with time and age. If you feel that she's living in unsafe/unsanitary/hoarded conditions in her barricaded home, you can call APS and/or the local police to do a wellness check on her. In fact, I think I'd call the police first b/c APS has a history of doing SQUAT these days until and unless an elder is literally living in pure squalor and starving to death in their home. The police can and WILL gain entry to her home to have a look around before they tell you 'she's perfectly fine' and they can let you know what they find when they DO go over there.

Put aside your feelings that mom despises you and realize it's dementia and/or mental illness at work here. Obviously the dementia can't be all THAT bad if she's still able to shoot of nasty-gram emails, however. I dealt with my mother for well over 6 years with her dementia, and she said some of THE most horrible things to me you can't even imagine. But what I figure is this: I lost my 'real' mother long before she left the Earth; I lost her to the disease of dementia which robbed her of who she once was & any logic she was once in possession of. The dementia turned her into a suspicious and nasty person who insisted the world was 'against' her and nothing would convince her otherwise. She's at peace now, and for that I'm grateful.

Until your mother arrives at perfect peace, have the police and/or APS check on her to see what's going on inside her fortress.

Wishing you the best of luck.

Three possible strategies:

1) Ask for a police or APS check on living conditions.
2) Contact the Condo owners/ managers and suggest that they do a check on Condo condition.
3) Let mother continue to call the shots.

The chances are that you are right, she won’t let you or your brother in because she’s ashamed of her living conditions. It doesn't necessarily mean that she despises you.

I’m so sorry, Brdion. I haven’t experienced this myself but I have known other people with a serious mental illness and I can easily imagine how horrible it would be if the sufferer were one’s own parent and their delusions included you.

I certainly hope others might have more insights or experiences with this kind of scenario to offer you. Best to you in this heartbreaking situation.