You should not use a regulator, it will limit the amperage and the servo will not like it. A 5 cell battery is fine.

If you stripped the plastic outer portion of the M/P gear you had to have put a tremendous load on it which could also be detrimental to the motor.

You should never give it to your LHS to send it back for you. If you are in the US or Canada our turn around time is about 5-7 working days plus shipping time. So the hobby shop may have waited a month before they sent it in.

Mike.