A assuming your buyers had a typical inspection contingency, which it sounds like otherwise you' shave no need to negotiate, the buyers can unilaterally back out without the need for further negotiations, let alone you didn't offer to fix the issuues, it issue a credits instead.  

Since the properties are valued over 1M however every % counts so by shaving off 1-2% this could make a big difference.

Spray foam in the rafters will probably do it.

Then spray foam insulation in holes to prevent rodent entry into the wall.

Since we were not doing any site work, we installed the pump and tank system, along with a fire alarm system.We also needed to get spray foam insulation installed in various areas and with the extra demo, we now had to board and plaster even more of the space, as well as replace the blown-in insulation that was lost.The final delays towards the end of the project were mostly related to some quality issues from the prior three carpenters and plasterer, so add another few weeks there!

Using weather stripping foam to seal one attic access.

I guess the first, and honestly, most important thing, would be to shave that mustache.

Maybe try and shave a few grand off other scopes to help offset.

I would try my best to hold them, but if I were facing default, I would try to liquidate or shave my personal expenses down as much as possible.

I'd look into modular construction, there's alot being done in commercial, as well as housing, also steel buildings, maybe tilt up concrete, helping to shave some time off.It seems to me it's an area where you need to think outside the box both time and weather conditions.