Recently confirmed this. Exit stamp is not needed through the E Gate. You will receive an entry stamp when you show your multi re-entry permit, on arrival back into Thailand.
No probs, I have had less than, 30 days and more than. But it is usually 30 days. That is the norm. :) They are a law unto themselves....555. I did the same as you; got a Single re-entry, returned before the consideration date and used it. Got the rest of the time stamped in, and got a multi re-entry.
That's seems exactly what you asked. As Graham said. 1000 baht. Re-entry. Get extension. Another re-entry. Immigration can put whatever date they like for the under consideration period. Sometimes less then 30, sometimes more.
I would recommend to you, in future, that whenever you get an extension of stay, get a multi-re-entry on that same day. This is what I do, so I never forget.