I've always thought that the mantling process seemed a bit arbitrary and easy (at least mechanically) fix to some of the complicated history of TES, and I don't mean to get my hopes too high but I had a thought that would at least make sense to me. Is it possible that the whole idea isn't an option for anyone who can "walk the walk and talk the talk" well enough but rather almost like a law of conservation in the TES universe?

To be more precise, at the end of Shivering Isles, the role of Sheo is vacant so the Champion takes that up. Did the Champion take end up as Sheo just because he was really such a perfect fit, or is it more so because someone needed to satisfy the role, and the Champion was a good fit?

Or perhaps for Lorkhan. He is dead...sort of, but allegedly his power still floats about. Do the Shezzarines truly embody Lorkhan's role perfectly, or is it simply they are tapping into that power/godhood by acting out his role since "he" isn't there?

This, for me, spells out a system where mantling can't just happen any time, but only to replace cosmic vacancies. A god (i.e. Lorkhan) might die in the sense that there is no direct entity enacting that role and thus that power, but the power is still there. So when someone enacts that role, then they get that power, but only in the cases that the role is not already completely filled.

Is this a plausible understanding?

I'd say so, yes. The steps are never exactly the same, they just have to be close enough to count.

http://imperial-library.info/content/morrowind-thirty-six-lessons-vivec-sermon-eighteen on the issue: