It uses the same bone-heat idea, but it never fails: visibility is tested against a voxel solid of the whole model and the maths tolerates broken geometry. It also handles imported rigs with arbitrary bone tails. On clean meshes with good rigs, quality is about the same as Blender's.

Yes. Loose pieces, split seams, holes and inner shells are exactly what it was built for. Pieces that touch get matching weights, so they move together.

Yes. Bones are run joint to joint, so arbitrary imported bone tails don't matter.

Yes. Fix Unweighted Vertices gives weights only to the vertices that have none and leaves all other weights alone.

Auto Weights replaces the weights of deform bones on the selected meshes. To keep most of your work, use Only Selected Bones or Fix Unweighted Vertices. Ctrl+Z always undoes.

No. It's pure Python and runs offline on Windows, macOS and Linux.

Yes. Max Influences (4 by default) and normalization are applied automatically.

Yes. What you make with WeightMend is yours.