BUG: MultiIndex truncated for some non-list arguments
closes #14794.

WIP.  Discussion/suggestions welcome.


pls add tests first. just make the simple change and get it to work. then you can do follows to restructure if necessary / more clear. There are a lot of things going going on here (IOW lots of routines hit this), and lots of variations of accepted, so need to be quite careful (and more so about performance). Thank you.  I guess I got carried away.  I'll do a fresh start and open a new PR.