"And what are we talking about here, a seperate figure for every different variation or a seperate rig to cover ranges of body-types (e.g. thin, average, obese, muscular)?
Is there any conceivable way to switch rigs in a figure in a user friendly way?"

You can inject joints (and falloff-zones) into a figure's cr2 just like you can inject morphs.

So ideally, if you incejt an "obese" morph or an "emacicated" morph, you could also inject modified joints and falloff zones at the same time.
Same for clothing, of course.

But I usually re-rig all my morphed figures and turn them into standalone characters as I want the joints to be as perfect as possible.