I will refine it as suggestions/comments come in. One probable change is the mount holes for the arms; rather than one long mount hole for a pair of arms, I may change it to one per arm. Shorter screws are easier to find. Will also consider your suggestion about the mounts. I'm learning. It's an E3D hot-end compatible mount. Though, as noted, it's parameterised so should be easy enough to change things for other hot-ends.
The choice of springs makes a huge difference to the operation of this unit. You want them strong enough to keep the head stable, but loose enough so that you don't need too much downward pressure to affect the pressure on the FSRs. I'm still building my printer and waiting on parts, so would be keen to hear how this works for you, and make any changes required.