If you are having trouble with some of the mountings you could always cut up an old atx case to make your own mounting hardware.
Mechanical engineer here too. Open wall mount sounds really interesting. With all the extra room, did you happen to consider losing some fans and just using convection? I have considered this before using FTR in a wall cavity, vertically stacked, with an inlet grille on the bottom, and outlet grille above. Probably 2 tiers. You could establish your own chimney effect between studding, and hide all the radiators. I would've created my own face panel on the wall for accessible piping connections. But no/diminished fan noise, none if you water cool the whole computer. Only pump noise for mechanical parts if you go all SSD.