"My understanding is that the words "aish-das" at the beginning of V'zos HaB'racha are supposed to be written as one word as per the ksiv of the Mesorah. If they are written as two separate words, is the sefer Toah passul?"
My instinctive answer, based on hilchot kriat hatorah, is that a new sefer is not required for kriah purposes, but that the error should be corrected as soon as possible.
I was not able to find a mekor that brings this shaila specifically.
R' Moshe or R' Eli, do you know of any?