I've been trying to understand why all the witness marks are necessary.

For example, why mark the transmission output flange in relation to the driveshaft? I'm going to replace my transmission so obviously this mark will be lost.

The FSM also shows to mark the flex discs. Once these are replaced, that mark is gone too.

Wouldn't the driveshaft be balanced out of the car without flex discs and tail piece?

Also what type of self locking nuts are on the flex discs? Can these be acquired at home depot?