Sharing animations with another model with different skeletons. Retargeting doesn't seem to be powerful enough to handle this one

Very lost with this one. Though I’m definitely still learning, I’m usually pretty good at searching for solutions to whatever it is I’m trying to do. That being said, I’m having a very hard time finding out even what I should be searching for to get where I’m going or what approach I should take with this stuff.

I grabbed some (awesome) animation assets from the marketplace a while ago and they have been pretty great for prototyping and such, but now that I’m moving past that stage I’m immediately stuck on this concept. The asset pack has its animations both for custom skeletal meshes and a “copy” of them for the UE4 mannequin, which again was really nice for prototyping, but they have some differences that are very difficult to overcome for someone without modeling experience.

The custom skeletal meshes have the weapons/shields/capes/whatever as part of the model with their own bones and all that. For prototyping I’ve been using the UE4 skeletons with retargeting (where necessary) and have attached a sword and shield mesh (no bones, just the mesh) with socketing. The issue is that while it gets the job done, it doesn’t look as clean and, the big problem, some of the animations have the weapons leaving the model (throwing the shield, tossing the sword, etc).

I’ve tried looking into swapping skeletons with blender but that seems to be WAY more complex than a copy/paste scenario, and “skinning” doesn’t seem to be the answer, either, at least with my cursory knowledge of what that is. I’ve also looked at adding the sword and shield to my new mesh and adding bones for retargeting just as a workaround, but again this is a bigger job than it sounds like and I’m not sure it would even work.

What I’m asking is simply what discipline should I be looking into, here? I fully understand that this concept probably cannot be fit into a forum reply, I’m just hoping someone here can briefly explain what it is I’m trying to do so I know what to research to get this figured out. I’ll link a quick pic here to show what I’m talking about. Please forgive the guerilla MS Paint editing.