VorGuy's mother made & hung curtains in his house (master bedroom) before we were married - but he expected that and knew the curtains were coming as he had asked for them (broke college student who needed curtains for his bedroom in his "new" house - he was 22 and using the GI Bill after Vietnam - it would have taken a long time for him to buy any kind of curtains).
We lived with those curtains until we moved out and rented the house - for all I know the renter (who bought the house from us about thirty years ago) still has them in her bedroom. Or replaced them after buying the house - I don't know nor do I care.
But - unless your MIL is one of the designers on a tv show whose taste you've enthused about (there are a couple of designers on HGTV that I would LOVE to have redo my house) and you haven't ASKED to have the house redone - then the boundaries that have been crossed are way, way too far out to be believed.
Seriously - her MIL is completely out of touch on how to treat her adult son and his SO - what would she have said to HER MIL thirty or so years ago if the same thing had happened to her house with FIL?
The keys need to be changed, the moat needs to be stocked with alligators and piranha, and MIL needs to have everything that wasn't agreed to (the pictures that were mentioned) taken off the walls, the throw pillows off the sofa, etc. and taken back to her.
Going through the underwear drawer, the laptop, the nightstand, and tossing out the DIL's personal things (it sounds like some VERY personal things) is the kind of thing that STALKERS do! NOT your SO's mother!