Tiles blown in shower, advice needed please!

Hi all, I’ve removed some blown tiles in a shower, water had been getting in and some has come through to the downstairs ceiling, so I’ve removed the blown tiles.
I’m happy to tile this myself but when I’ve removed the tiles, there is a sheet of ply behind which had had a skim of plaster, then tiled on.
The plywood is damp, but good apart from the very bottom near the shower tray, and has a couple of mm of flex on it in the middle.
I could really do without ripping the whole shower out. I ideally want some advice to make this sound and waterproof as is.
Any tips on getting old adhesive off tiles would be great too. Thanks!