This is what I put under my board, you may use more or maybe less if you like, but this is how I do it. I put my backplate under the padding sheet and through this sheet and shop toweling so the towel is against the board
This is how I insulate the around the socket area and up to the CPU, went a bit overboard into the mosets and such this time you really don't need to do it that much (I think this image was from my first time)
I do also take off my socket latch and put thin strips of padding inside it around the socket inside the latch itself.
And I fill the small square on the backside of the board under the socket with eraser as well, both of which are up to you and not necessarily needed.
Memory and socket area insulated for this run, I use much less eraser now
On top of the eraser I put a layer of toweling around the CPU area
Then a layer of padding above that, and one more layer of shop towel
Notice the multiple layers of padding and toweling
I also use a rubber band around my paper towels at the top.
As for everclear, it looks like you used too much is all. I use about 1 to 1 1/2 inches in the bottom of the pot is all, then SLOWLY add your dry ice, VERY SLOW or it splashes everywhere. You can put in dice first if you want to let it cool down a bit, then slowly add in your shot or so of everclear, whichever you find works best for you. I think your temps were not so great because you used way to much everclear is all.
And just so everyone knows, because I swear by it, everclear will get as cold as acetone if not colder, you can run 68-69C for sure
Some boards BIOS or sensors will not fully register cold temps, so your BIOS may only show -10C to -5xC and windows utilities may only register above 0c or just below.
Hope this helps you better insulate next round, and get some better temps!
Sorry to hear about your Gigabyte board! Are you SURE you let it dry long enough? Did you put a fan on it overnight or anything like that?
Pm me about it if you feel it is dead, maybe I will take it off your hands if you are going to toss it