I would start by calling all of the HO's and explain that the 10k charge is being levied against the GC, not the HO. Explain the reason for the 10k backcharge to the GC due to the fact the drywall contractor has reduced the amount of work he is performing thus increasing the amount of work you must perform. At this point the HO's will have a clear understanding of the charge and subsequently they will tell the GC they will not be paying. Period.
I've never heard of the drywaller's not cutting out the MEP openings. If the drywaller changed his scope of work with the GC and the GC failed to inform you, then it looks like the GC is eating this one.
Verify that the GC did not include this additional work in your scope of work. Otherwise, you may have to eat the charge.
I recently had a spat for a GC for a similar problem. He wanted to backcharge me for his time/material to uncover some undercabinet lighting whips the drywaller had buried. I left the whips hanging out of the walls into the room and they pushed them back and covered them.
