Mapei renovation screed is good, make sure you use correct primers.
I also use foam and cut excess off after SLC set.
Mapeker isn’t very user friendly imo. I would use an S1 adhesive from any repuable manafacturer - your local tile shop should carry a few options. Does it have to be rapid? A...
The Mapei kit bucket with mapegum, tape and primer is good and readily available in screwfix. I like the 2 part ardex one too but it’s not as user friendly.
Put the tray in tight against the squarest corner and pack the 3rd wall with board, either dot and dab or double thickness if tacked. I’d use a waterproof tiling board like orbry but plasterboard would do. Tank before tiling.