PVA on pool paint

It's better use special staff for plaster it's green star for plaster

I would use something more like thistle bond it the green stuff with grit in or the tiling equivalent as provides better key and sealing properties I find just paint on day before you start but I would do more research on background surface first to avoid any bad reactions

Pva is a mixture for area off suction or a pure mild glue for wood
What I look for is a waterproof design like a slurry mix that will help surport your tiles but also seal your backing coat ..first get a power washer or steamer remove ass much access grim .chemical as possible then fill all sensitive areas drains and water inlet pump then cover with a slurry allow a few days to dry or cure then applie your tile then as with any new aqua construction fill and empty pool a few time to cut down any chemical change that can cause thank you

PVA is not suitable to cover over paints. And also not used for rendered services.

I wouldn't. contact a supplier for pool maintenance materials they will have what you need