Tilerite Therma-board and wedi backer board

Yes you can use either of these but make sure you pva the surface before tiling

In theory you can use any sort of boards. What it is important is the preparation of the wet walls. You can use products sold by Mapei or Bal that are like a paint that you brush on the plasterboard before tiling. Wedi boards anyway are sold in different thickness. Plasterboard is usually 9mm or 12.5mm. Wedi boards start from 9mm, 12mm. So you can use wedi board instead that normal plasterboard. I personally fit wedi boards and use mapei waterproof kit that it is composed by a thick paint and a tape that goes into corner to make the wall water tight.

Never pva a surface for tiling. Wrong practice. You can use 12mm hardie backer cement boards on the wall and tile directly onto, you can use BAL PRIMER for any walls that need to be free of dust and dirt so the bond is strong not pva. If it’s for wet room or shower area make sure the corners are taped and waterproofed with with a waterproofing tanking agents found by BAL products.

Personally I prefer hardibacker boards no need to seal or prepare boards before tiling, these are readily available at Topps tiles in 6mm for floor and 12mm for walls.

Yes they are both the same thing. Do use PVA it will lift off. No primer needed if you are tiling directly on top.