My mother gets obsessed with one thing, whips herself into an angry rant and asks the same questions/ talks about it for hours.

MidKid58, My mom did the same thing. It was so bad that I got phone calls many times during the day, her husband was dealing with it and finally had to put her in Mental Care to get medications to keep her calm. This worked as long as he could get her to take meds. Sheriffs dept was called because she pulled knives on him. She did not realize he was her husband. Thought he was just a friend and would not give her keys to her and was keeping her hostage, she actually called the police on that occasion. This happened at least 4 times before he realized he needed to put her in a Memory Care Facility.

This sounds like my MIL, but she does it all day long--meaning, she's able to be as angry at 11 am as she would be at 6 pm....

She had a lousy marriage and it ruined her life (basically this is ALL she ever talks about--every conversation turns into a rage fest of poor-pitiful-me--and b/c nobody buys into it for one second, she never stops. I think even IF we agreed with her, she'll still be stuck in this mode)

She's been like this for, oh, 15 years and it IS getting worse---but she is totally alone and something of a hermit--rarely leaving her home. She's driven away her 2 sons (one of whom is my DH) and his older brother. Only SIL really sees her---

I don't know how you could even go about having her evaluated...it's her actual personality, I think, but I have noticed that when we happen to be together as a family, it is MUCH worse. (Saying that, I have to back up that we see each other as a "family" once a year, if that.)

What could be done for her, at this point, anyway? (just turned 89) And she plans to live to 100. I'm sure she will.

Addictive phone games are my best friend for those periods.

The game is all the mindless obsession, repetitive action she seems to need but with structured clear goals that are constantly re-stated so it keeps her on track.

If you can find a game that works, then buy all the locked content and let her at it. Three to four hours at a time on the game during the p.m. And since video gaming is a normal behavior, we can call it a hobby when in reality it is a redirection of aggression and confusion! You can ask how the game is going during commercial breaks.

Gives her something to talk about and share her achievements, and it is a very simple little world with simple actions that her working memory can handle even though long term and midterm memory might not be working.

If she seems totally unable I can say "hey, what level are you on??" and she starts opening up the game and being proud of being level 85 and having got all the items ...even though she got trapped in the car at the supermarket parking lot today because she forgot how car doors work.

She is sundowning and there are meds for this.

Here are a couple of links to check I picked up from another post.
They might help explain what is going on.

https://www.agingcare.com/articles/autumn-and-sundowning-210080.htm

https://www.agingcare.com/articles/how-to-cope-with-sundowning-tips-from-family-caregivers-200000.htm

Before TV time, use ear plugs. And think about her moving to a facility. Do not risk your marriage. This really can tear you apart.

Google " sundowning"
Get her seen by a geriatric psychiatrist as soon as you can..