*But wait: how does that affect the normal function of the hip actor, though? Does the figure still follow hip (or rather, hip replacement) movements?

Yep,  but forget about walk designer you'll have to create bvh's. 

*How is the ability of taking poses from 'standard' figures affected?

I never recommend taking poses from standard figures and applying them to your own.  Too many mesh modifier tricks in them.  reversed parenting for scaling is one example.

What happens if instead we call the original hip hip and the replacement pelvis?

Then you will not be able to isolate the hip and you'll end up with an added abdomen group.