I've heard conflicting answers on this and never had to deal with it personally (so never had to research too deeply).

If a MMLLC elects to be taxed as an S Corp, when it revokes the S election (or when an ownership change results in an ineligible shareholder), does it revert to a C Corp or back to a partnership?

Back when I learned this you had to file the 8832 first (electing into corp treatment) and then the 2553 (to get into S).  So I've always assumed that when you undo (or blow) the S you step back into the C.  Then if you wanted partnership you do another 8832.