A-1: because it’s something else than a humanoid for example, but I’m sure you were talking about humanoid.
A-2: unreal don’ let developers chose their price, for consistency they do the $.99 thing.
A-3: if not humanoid then there are not so many choices besides exporting to fbx the Skin pose version and adding some new animations in a 3rd party program.
B-1&2: not sure about the programming workflow in UE4 but indeed if you have a character with optional meshes that you can swap out or show / hide then in UE4 it’s best practice to only export individual pieces that you can assemble via Blueprint or C++. At least that’s what I do for my characters submitted for UE4. All extra optional meshes or pieces of armor, in my case, are usually skinned to the rig. I set up a showcase blueprint with the “Set Master Pose component” for the extra meshes and voilà…
Now thanks for your input but that wasn’t really helpful unfortunately… 
UE4 has some limitations and constraints regarding humanoid that are not extremely clear, leading to confusion and mistakes… Also their pipeline is limiting, constraining artists to use Maya or Motion Builder for rigging and/or animations. On my end I use 3DS max with Biped, It’s a lot of extra step to rig an additional skeleton to the Epic Skeleton… The way i figure things at the moment is rigging with biped, create another rig with basic bones snapped to the biped bones, full of constraints (look at and position) to be driven by the biped one. Skin the model onto that second rig. Start animating in 3DS max with the biped and export the animations from the second rig… Also Biped doesn’t allow you to rename the bones the way you want…
Another way I though of would be to rig and animate in 3DS max with Biped with almost the exact same hierarchy ( here you’d have to unlink pelvis/hips from the biped root and parent it to a new root bone at (0,0,0), export the thing to fbx with the whole animated timeline. reimport that fbx, rename the bones to the Epic Skeleton naming convention and finally re export each individual animations meshes etc…
If anyone else know of a better workflow I’m all ears. Really want to get some stuff done and it’s pretty frustrating to experience delays because of what I consider constraints that should have been smoothed out …