1. No, but it is easier to do in a complex single. (personally I think for lazy 8's)
2. You can teach zero hour Multi students. But good luck finding them. Usually it starts off as single commercial then CFI/CFII. This way you can build time teaching students. Then as you are building time, you can do your ME and MEI add on. By the time they are done with their commercial single, then you should have all your qualifications to teach them.
If you are going to do this route, do your instrument add-on with your Multi CFI. Also, this check ride usually has to be done with the FSDO.
3. see above