Tile Failures and Problems

Background:

House was completed in early 2020. We have a large master shower, roughly 5' x 8' with a drop floor to accommodate a curbless shower. There is also a large slate bench on one end of the shower. The contractor painted the entire shower surround with redguard from floor to ceiling prior to installing the pan. I don't believe he pre-sloped the subfloor prior to installing the redguard, and I'm not sure how he finished the redguard at the drain flange.

2 years later, now the dining room ceiling directly below the shower is wet. Mostly focused directly below the bench, which makes sense as the grout joint between the bench and the back wall shows a crack (which shouldn't matter if it's sealed well below) and our 3 year old daughter likes to stand on the bench with shower water running on it, so the bench has seen a lot of water over recent months.

I expect that the main failure location is below the bench, however at this point I do not trust any of the installed pan work and am preparing to remove and replace everything, and I'm looking for some advice on how far to go with the replacement work.

Repair:

My thought is to go with Kerdi liner system for the entire pan and surround, plus the Flo FX flange.

One question, is how far do I need to go with demo? If I leave the lower redguard pan in place, am I going to create a mold sandwich? What if I do a few core drill holes to open it up to below? I'll be in this house for many many more years, and this is going to be a massive repair job, so should I even be considering salvaging the pan, or should I just go strait down to the wood below?

With the no-step threshold I'm concerned about water traveling out the threshold. Outside of the shower is also tile with heated floor. The contractor did some sort of light waterproofing below the floor heater, but I'm not sure what it was, nor do I have full faith in it. I'm wondering if it would be a good idea to run a strip of something like TROBA-Plus along the shower entrance, with a 2nd strip running from the first, towards the drain? This could effectively intercept any moisture heading out the door, and give it a path to get to the drain. Or is there a better product to effectively do the same thing?

Any other advise on technique or materials would be appreciated.