but I weep for the younger generation..... and I'm not that old...
The only thing left on your scalp is the residue of impurities which might burn if you have an industrial furnace.
It means short of jumping inside an industrial furnace you are safe.
no, it's not flammable. It's gone completely. Trust a chemist.
Whether it's dried or not you should shower and wash your hair more so than you usually do. Then, when your hair is dry and you're ready to straighten it, have a bowl of water or the shower head running just in case.
Alcohol does not dry. It is a vapor at room temperature. After this long your head is fire safe :)