you needed a modified adhesive, that means one that is flexible. the same for the grout, re-grout using a flexible grout to absorb any movement in the floor. as for the adhesive used, it should have been in a bag not a bucket
no you can't use sand and cement, because it won't bond to the porcelain tile. you need a modified adhesive for porcelain or an adhesive with porcelbond in it.
you dot and dab plasterboard adhesive every 4 to 6 inches then offer the plasterboard up to the wall and check with your level to make sure it is plumb
i'm sorry to say that you will have to live with the scratches or replace the tile in question. as it is a ceramic tile it is the glaze that is scratched and there is nothing that will cover the scratches.
sorry i couldn't be more helpful
there is no such thing as waterproof adhesive. all adhesives are water resistant not waterproof.it's alright using aqua panel as long as you seal the joints
bal fast flex is the best and you can tile straight on the floorboards. personally i would overlay with 15mm ply on chipboard and use a two part adhesive. bal fast flex is the best but as you said it is pricey.
you can't tile on paint without mechanically removing it as to british and european standard. so my advise is to use an apd primer which you can tile directly on
hi rach
to stop the dust use a mixture of pva and water. then just pour on the floor and sweep it around and this will seal the floor and stop the dust