You can use c++ to split the mesh, sever the bone, do whatever transform, and re-attach the bone.
And your quip for #2 makes 0 sense.
Further reply and the implication you would need to edit the actual asset makes even less sense.
Add bone modifiers to your animation blueprint. See how they work.
Propagating/not propagating was a somewhat recent addition (2020 or so).
You don’t even have a need to loop the chain to reduce the other bones anymore.
You are honestly just both off topic and overcomplicating the topic for the next poor sap dragged in from a google search…
Though one thing is true: 300 meshes is nothing. If one is scared to edit stuff because 300 is a lot, then perhaps hire someone specifically for the purpose… lots of potentially jobless artists out there that can probably do the edit in less than an hour…