I can't really answer all of your questions, but just to clarify about your mirroring. The mirroring state will not go to suspended unless someone is issuing a command to suspend mirroring. My bet would be the index rebuild process is suspending the mirroring and then resuming it after it is done.

As for having 3 databases mirrorring - shouldn't be an issue at all. I mirror 14 databases from CA to TX with no real issues. After an index rebuild or a purge operation, the transactions for the mirror queue and can take several hours (sometimes days, depending on the changes) to catch up.

My guess here is that your snapshots are causing the majority of the problems. Do you keep multiple snapshots? If so, how long are you keeping them?