Tiling over latex paint that has water proofing membrane
I have applied thinset over paint many many times.  I mean they have waterproofing paint (redgard) now and you thinset over that.   Now the question is did the paint cure and does it have a good bond to the surface?  If you don't know that then mix a little thinset, put up one tile (where you will eventually want it), and test it out.  If it is solid the next day then you are good to go.