THAT "Tile Basin" product is "one poorly designed product" in my appraisal. So-be-it, it's done now.
The mastic was the big mistake. The instructions say "use thinset" and you didn't do it.
The next problem is the flimsy basin itself. The instructions say to install it in thinset but the unit may have a little warp in it based on what the one in the website photo looks like. The basin pictured in their website photos looks sort of cheesy. They show it being driven over by a car. Unless you plan on driving your car into your shower that's a stupid way to try to sell a shower receptor. Thinset won't take out the "quality control warp" that I suspect your unit has and thinset won't support the resulting voids that may occur under the unit. Movement in the substrate means loose and cracking grout and tile. Period.
Take out all of the tile and remove all of the AcrylPro. Check the basin for flexing. If there is even the slightest flex (and I'm betting there is) then remove the basin. It should be thrown away at that point and another method adopted. If you try to reuse the basin IT MUST BE FIRMLY INSTALLED WITH NO FLEXING.
The tile is then installed with thinset according to the instructions that come with the unit.