Confused about Full-Subtractor truth table
"How does a difference of 0, and borrow of 1 represent a -2? Makes no sense at all."

That makes perfect sense, because a borrow is in effect a -1 in the next column (to the left), which has a weight twice the weight of the column we are working on. Hence when a 1 in your column represent 1, borrowing (= subtracting) 1 from the next column (to the left) represents -2.