Well, I am glad to hear that I am not the only one who has had this truly , as the OP stated, MONUMENTAL F-UP.
I did the exact same thing, when I was doing three brews in one day. I actually caught mine during the mash, but was totally vapor-locked as to what to do to fix it. So I just mashed it for 120 minutes, and collected about 2 gallons of wort. I can't remember any specifics of OG or FG, all I remember was that the beer was very grainy in the flavor, and I didn't use near enough hops to give it any other levels of flavor.
The next day I actually thought what my fix would have been.........
Get out the stick blender and go to town on the mash.
But, I digress, that one is just chalked up to being a *******, trying to do three all grain mashes in one day.