There's nothing wrong with combining the geometry even though it doesn't make a continuous mesh, we do that all the time. Simply combine them into a single object. Not judging here, but is this your retop you're showing? That could be cleaned up and made much more efficient for animation.
The rigger will weight the mesh to the skeleton using 'masks that are tied to the joints', aka skin weighting or just weighting. So the rigger could go in, place a joint, and paint the weights. So even if the mesh is combined, they will be able to animate them separately. That being said the weighting 'mask' doesnt need to…
Well having a watertight mesh has advantages. For example the jacket/pants that you have intersecting right now, if the weight painting isnt done correctly, they could 'break' during animation. Its not a big deal if the intersecting parts are midway on a bone, for example the boots/pants area, if they were intersecting it…
Depends on the complexity of the animations/rig. If those objects dont need to animate, you can optimize as aggressively as possible while aiming for the quality bar that you guys have set. Which could mean a very low poly proxy mesh that is water tight. A watertight mesh, combining everything together and getting rid of…