25 years ago a friend of mine (who was unemployed and needed the money) tiled my bathroom with Villeroy and Boch tiles, all four walls, floor to ceiling. He tiled directly on to painted plasterboard. From memory he used a tubbed(?) Bal "waterproof adhesive" and waterproof grout. I remember having a row about the adhesive because he had a mate who was supposedly a professional tiler and he'd told him you didn't need waterproof adhesive as long as you used waterproof grout. I bought the waterproof adhesive anyway.
I suspect he used the 5-spot method of applying the adhesive too.
Since about 3 years ago we fitted a shower and that's become its main use. Last month I ground out all the grout and trims, re-grouted and replaced trims. Point is, the tiles are still on and nowadays it would be considered a bodge job. It was probably a bodge job back then too, for all I know
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I think what I'm trying to say, completely unqualified, is that it'll probably be ok and if it were me, I wouldn't be ripping tiles off and re-doing it.