Floor needs to be dry 100% then Ditra Matting installation and of course tape the joints. That's the best you can do as a tiler. Send us some pictures. Good luck!
That gap is way too much for a shower. If the shower tray is solid, you can remove the last row cuts and redo them to eliminate the huge gap. You can silicone after the tiles are redone. That would be the best for that shower. I would waterproof the area as much as possible. Send us some more...
Install anti crack membrane Ditra matting, any brand would do.
https://www.tilingsuppliesdirect.co.uk/product-category/wetroom-systems/decoupling-membranes
Schluter Kerdi 200 like products are perfect for the application as long the wall is perfectly sound, make sure you apply the corner band up the wall/corner. Send us some pictures. Good luck!
It looks like that floor was designed for vinyl flooring. The green tile is vinyl. Get a dehumidifier to dry up the soil then you can proceed with the concrete and tile. Ardex products are good. As far as the bitumen blackjack goes, make sure Ardex NA is compatible with bitumen. Send some...