I've worked in the building trade for 15yrs as a bricklayer/roofer and in that time I've picked up and learned other trades such as joinery, plumbing, plastering ect. all handyman jobs welcome.
Firstly take the floor boards up around to fix the leak on the toilet once fixed replace any damaged floor boards then once the leak and boards have been mended it'd be a case to take down the damaged plaster boards on the kitchen ceiling replace with new then skim the ceiling with plaster
I've papered over both silk and matt emulsions in the past and all went well on some cases depending how bad the wall is i've had to sand silk emulsions walls down but matt emulsions if the paints cracking away scrap it off then wipe down with damp cloth not depending how bad the walls been painted
Yes you can tile over tiles but best to stagger the joints with the existing tiles that are tiled over to make the bond stronger
I'd remove any wall paper before skimming it but i've heard that if you pva bond the wall the night before it can be skimmed the next day