Insane? Yes - along with lazy, careless and anything else you want to throw in there.

I wouldn't touch the mirror in any of my cameras with a micro fiber cloth (that I use all the time on my lenses), which is much better than a cotton shirt! I highly doubt that he would do anything like this to his own personal gear. May as well throw it in the river to see if you can get the dust speck out !

Bring it back into the shop and talk to the manager. Four to eight weeks is probably not out of line for a repair, but given the circumstances maybe the manager would give you a loaner for the way your gear was treated. Oh, and have them replace the mirror while it's at the shop (at the store's expense). Point out any damage on the mirror (check it carefully with a loupe) as the reason you want the mirror replaced. If there's no visible damage I'd still ask them to do it but probably wouldn't push it as much especially if they gave me a loaner.