It has been a few years since I instructed but i have never heard of a MEII. CFI-I is good for both multi and single ( and I believe heli too) as long as the person is rated in the a/c you dont have to be. Put simple, yes you can instruct in a multi with out an MEI as long as the guy already has a multi ticket. You dont even have to have a CFI just a CFI-I. and yes you can get a CFI-I with out a CFI
Best part is the MEI ride is like a glorified ME ride without the Instrument approaches.
One of the easiest checkrides I have ever done. Very involved traning though, that is if you get traning with the right MEI.