Sorry you are going through this. It really does not have to be this way However unfortunately the pool industry seems to have more than it's fair share of hacks. I was looking back over the pics trying to come up with a reasonable approach to this mess and discovered something even more concerning. Looking at the original dig I was having a hard time believing that the sand layer just happened to be at the correct elevation for all the walls. Turns out it was not. In one pic I can clearly see the spa shelf has a loam layer under it and a dusting of sand on top. That is where I would put the brakes on and reassess letting these clowns finish your build. I do not know of any soils engineers that would let that one fly. Perhaps @MinerJason could chime in and give you the likely hood of your spa settling. Sorry to increase your stress level but I really think you should get a few more opinions on letting this PB continue. @jimmythegreek builds pools in your area, Hopefully he can chime in as well.