Decware makes diffuser kits and PI audio makes some cheap diffusion units that may be worth a look. Can't say how they work as I have not used them. We know how smart Steve Deckart is on this stuff, and Dave Elledge at PI Audio is a serious music head too, and I can attest to the quality of his Uber power filter and his power cables. Very good. The stone wall looks like a pretty difficult diffusion solution.
Rock wool seems to be one of the chosen sound absorbing materials so should help keep your sound room "contained." If intense isolation from the rest of the house is critical, I guess you have seen what folks will do with offset stud walls that are vibrationally isolated, hanging ceiling structures, green glue for the sheetrock, and the like? With sheet rock interiors though, I guess you are planning bass traps and internal absorption within the sound room? I am of a mind to do the gross stuff first and go from there, but I may just be lazy and resistant. But I suppose there is merit to the flexibility of especially first reflection stuff. Like getting your bass traps sorted out during construction and then go from there based on sound preferences. The fact that you can do built in is a huge advantage though for visuals so you can build in diffusion and absorption, and hide big bass traps and all. Wow. Does make design important!
If it is about the sound, shorter seems better for ICs and less so for speaker cables as long as they are big enough to carry the sound with little loss. I see you have styx, and being 8 gauge and silver on copper, they are big cables. But then, if you can put the stuff closer to the speakers, and isolate it from vibration, shorter everything seems best.
It is hard to comment without knowing what you are doing and planning. I don't know, impossibly to say, but this could be part of why the big room heads have not chimed in.