Is it possible to re-tile over existing wall tiles?

Yes you can but its not something i would recommend.You will need a tiler to take a look at the condition of your current tiling/walls etc but cutting corners rarely produces a quality finish.

If the existing tiles are solidly fixed and level, and by going over them you are not losing too much of any existing architrave, or window reveals. Then you can tile over them, however they must be primed first with a good quality pva primer.

yes

Hi yes youcan if the tiles are sound make sure they are free from dust or grease do not use PVA as this only forms a skin on the tiles if replacing with ceramic just use a pre mixed adhesive such as Bal White Star.