Any armature with a skinned mesh whose deform bones form one hierarchy: UE5, Mixamo, CMU/BVH, most game rigs. The bone mapping reads the common naming conventions; any slot can be set by hand. Rigify as generated needs a deform-only hierarchy first.

Bones without a slot (twist bones, extra spine and neck bones, metacarpals) stay in their rest pose, fingers keep only their curl, limb twist is shared out by Unity's own rule, and moved bones stop moving without Translation DoF. Check Rig lists it for your rig, bone by bone.

On CMU motion capture (15 clips) the skin's typical frame went from 33.8 to 16.6 mm, better on every clip, each measured with the corrections learned from the other clips. On the UE5 mannequin with only four clips: idle 70.5 → 43.3 mm, walk 61.8 → 37.9 mm; no clip worse. Your report gives your numbers.

Yes: the rig learns from your character's own clips and then runs on any Humanoid animation played on it, never turning a bone further than your animations did. The measured numbers are for your own clips; for animation from elsewhere there is no original to compare with.

Limb twists move no joint. In-between bones (spine, metacarpals) move the joints above them the way your animation does; tick Keep Avatar Joints and every avatar joint stays exactly where the Animator and its IK put it.

Unity in this version.

No.