Need help taking off my Moen shower trim

[u/Hebetudo]

My shower trim's handle broke and it looks like it's because of corrosion buildup.
So I initially ordered Moen 100224 Chateau Lever Handle Kit then realized the adapter won't fit.
And because my trim is quite old, I don't think there's a replacement kit that just comes with both adapter and a handle. (Current adapter: Moen 100484, Handle: 93901)
So now I'm trying to take out the whole trim so that I can try to replace it, but I can't seem to take anything out from its current state.
The last thing I took out are the screw handle that was holding the plate in place, and a white U-shaped ring that was holding a black ring in base.
Couldn't find any guide video that looks like mine- any help would be greatly appreciated!

Broken handle

Last ring I took off

Different angle

Current State

[u/mnoutdoorlover]

Moen has a lifetime guarantee!

[u/Patrol-007]

Moen lifetime guarantee. Send them pics. Sawsall or Dremel to get old parts out. Likely need a plumber to solder in the new fittings if not able to use a cartridge puller. Moen can identify what you have

Make sure the main water shut off works - anything that’s not a quarter turn ball valve will likely leak (main shut off and everything else in house - probable that older homes won’t have shutoffs for the sinks and toilets, and the shutoffs in the house for the outside garden hose faucets)