Well if you want to waterproof the area you are best of tanking it that way you will have no problems tiling over that. Mapei Shower Waterproofing Kit - https://www.NoLinksToThis/p/mapei-shower-waterproofing-kit/78484
The product you have used there would normally be laid under the screed, so no I would not tile straight on to that. Why have you put it down in the first place? Take it you had rising damp though the screed?
I tiled a job a few years ago were they used that same product, I still wanted to overboard with 5mm wedi but the foreman was adamant I tiled straight onto it. Never got any call backs, but Still wouldn’t use the product on any of my jobs.
I would not try and cut around the skirting just mark a line up the skirting where the tiles will come to and using a saw or multi tool if you have one remove the skirting so the tile will slide behind, much neater finsh in my opinion. I can’t see why you can’t tile straight onto the plaster board.
What I do is finish all the tiling leaving no more than a 2mm gap in the corners, grout making sure not to grout the corners. Then put a 5mm silicone bead down the corners. I use a Cramer fugi kit for the silicone.
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