How to waterproof a interior wall that is dry walled?

Flexseal or Polyurethane will work as a sealant.

Don't use Flex Seal. it breaks down over time and will peel in chunks. Three coats of a water based polyurethane would work. Just beyond the height of the spray. Vinyl wallpaper would work. Your flooring idea is great.

Staple heavy duty black landscaping plastic to it,cut the plastic long enuf to extend onto floor then outward about 6-12" it's relatively cheap and can be put up & replaced easily when it needs changed out . Make sure you overlap edges and then fold them over so it creates a sealed edge strip if you need to make it wider.Cover staples with piece of duct tape so if they pee on a staple it won't seep into the staple holes. Even if it's water proof it will eventually smell like urine so if you use this you can change it out frequently. I run a trap depot for TNR for Animal Defense League of Arizona teach people behaviour of house cats,strays & ferals. Buy commercial grade odor neutralizer from janitorial supply, it's expensive but works, the kind that's used to remove vomit,blood,dead smell,urine,feces.Put it on the wall before you put plastic up it will remove the scent already present.Use a black light to see where it is. It'll be florescent. Try to stay away from any cleaners with ammonia in it it only intensifies the ammonia in the urine.THX for helping animals

Isn't is true that neutered cats are less likely to spray?

They make peel and stick vinyl floor planks that don't need glue. They have a very sticky adhesive on them already. Peel the paper backing off and stick it on. One person job.

Clear Vinyl Sheets, screwed to the walls.