A slab floor is about as solid and inflexible as you can get, so that's great. My floors are concrete and I love not having a crawlspace.
I would remove all the shower stall floor tiles, and carefully inspect the liner (although water won't damage the concrete underneath), and then use mortar to create a better sloped bed right over the liner. You'll have to maintain a 1/2" or so thickness at the drain though, or it will crack. Make sure to let this cure completely, wetting as it cures for maximum strength.
Try for 1/4" per foot of slope. If your slope is close to that already, then I wouldn't increase it, but instead just concentrate on a really careful floor tile job.
Then tile right over that using polymer-modified thin set mortar, being sure to let the tile set for a couple of days. It's ok that the new bed and tile cover the bottom edge of the wall tile if needed, since it's above the waterproof liner. I would use 1" or 2" tile, or mosaic, it feels great underfoot. Something purposely smaller than the wall tile. Make sure to use the correct grout for the size of the joint you use.
If there's any doubts about the integrity of the wall tile, then it would be best to remove them as needed, and the liner, and do the reslope under a new liner. Then you can reinstall the wall tile as well. The problem with this is that new tile may not match perfectly.