May be worth considering taking the tiles up.rendering or screeding the bottom again and retiling using a good waterproof adhesive.Will take you a bit longer to do but the result will be longer lasting.You will still have problems with the paint flaking.
Have you looked to see if there is any reason why the tiles are coming off .I repaired one  a few years ago because water was getting underneath from a poorly fitted inlet/outlet pipe and the tiles were lifting in sheets