Corrective Smooth (Delta Mush) is evaluated every frame, is slow on dense meshes, and does not exist in Unity, Unreal or any FBX/glTF pipeline. JointForge turns its effect into ordinary shape keys driven by the bones, so the deformation travels as blend shapes and costs almost nothing at runtime.

It works on any armature that deforms a mesh through the Armature modifier, with or without Preserve Volume (mannequin with Preserve Volume on: worst 45.8 → 21.8 mm, typical 3.23 → 0.72 mm). Twist bones are handled — on the UE5 mannequin the hip and shoulder are bent by bones whose skin hangs on twist children. Control rigs are fine: when the deform bones follow a control skeleton through constraints (Rigify, game rigs) and the animation lives on the control skeleton under other bone names, JointForge finds those actions and reads the deform bones' final rotations; on such a setup the results were identical to the plain rig.

On the UE5 mannequin with four animations: the worst frame went from 41.6 mm to 18.6 mm off the Delta Mush reference, and the distance 95% of the skin stays within from 3.21 mm to 0.72 mm, with no frame worse than plain skinning. On animations the keys were not fitted to the gain is smaller (about 20% typical, 30–55% on the worst frames), so fit on all the animations you have.

One per joint, per bending axis, per 30 degrees of the range your animations use — 699 on the three LODs of the mannequin (233 per LOD). Joints that barely deform are skipped. Untick joints such as fingers to keep the count down.

Yes: Export for Unity writes the FBX, a driver table and a component. With Unity playing the clips imported from the same FBX, the component's blend shape weights match the Blender drivers within 0.004 on 99% of values. Unreal is not supported yet.

No. Remove Correctives deletes only the keys JointForge recorded as its own (they are named JF_), never yours. Its drivers are plain expressions, so they work even when a file opens with Auto Run Python Scripts off.

No. Blender and numpy, nothing else.