Hi yes you can tile over existing tiles as long as there well stuck and solid And flat and there’s nothing loose , as long as you use the correct adhesive, Get you adhesive from tops tiles or similar, I’d use a bal adhesive, and you would set it out like any normal wall , find the centre and cuts at either end .

No problem, tap/ knock the existing tiles first to make sure they are still bonded to the wall and not sounding hollow / loose, then use a primer (i use SBR) and apply with a brush over the existing tiles and let dry. you are now ready to retile. if however you are unsure of the material of the existing tile and the one you want to use then i would use flexible adhesive and grout to be safe

Yes you can tile on top of tiles, make sure the tiles are solid with no loose tiles. I would advise you use primer on the old tiles before sticking the new tiles on. Best I’ve used is ardex p4 primer.

Yes. You have to use special primer to go on top of the tiles and then use good quality flexible adhesive. As long as your tiles are solid, you good to go.

yes you can but i wouldn't recommend it

To tile over tile make sure the existing tiles are well stuck and solid before fitting. You need to get yourself a good NEAT primer and ensure the surface area is well covered. As soon as that is done you are good to go to tile just like any other wall, just ensure thickness of the tiles etc. Won’t be an issue if tiling round existing door frames etc.

Existing tiles need to solid to the existing background.
I use Mapei Eco Prim Grip Primer.
Which when dry leave a rough texture as it has a grit in it which helps the adhesive used to give a good key.

Best thing is check for any hollow tiles/ cracked tiles. If there’s alot of hollows I would recommend stripping completely as it could be a fail in the long run. If it seems solid you can prime grip the walls first(let it dry) then tile it as you would normally👍