PROVERBIAL BALANCE
Proverbial balance is not a term. It's a proverb from the normal human world. Well, the scales must balance is a proverb, which is why D'Hoffryn uses the adjective "proverbial." He's talking about bringing twelve people back from the death, he wants to balance that by killing off a soul that has the same netto-worth; the soul of a vengeance-demon would be that. I think this has nothing to do with the balance between good and evil, but rather with the moving of energy between dimensions. He must put just as much energy in, as he takes out. The soul of a vengeance demon would then have more energy than a human being. Anyhow, this last bit is a bit theoretical, but like I said, "proverbial balance" is not a slayer-term.
TEMPORAL SHIFT
Combining the definitions from the Cambridge dictionary of shift and temporal: "A change in position or direction | relating to practical matters or material things, rather than spiritual ones." The term is used in Angel's "Waiting in the Wings." Wesley says: "Well, ah, this kind of temporal shift can't just exist. It has to be maintained. That requires power and concentration. If we can overload him somehow, we might be able to slip back to the real world." A few lines before he explains the situation as follows; "He was obsessed with the girl. - When he found her with the other man, he went insane with jealous rage - pulled her out of time (slowly walks towards them) out of any reality beyond *his* theater, his company. He swore she would dance for him forever." In conclusion, I would like to argue that the man they call Joss made a little mistake here and used temporal as an adjective relating to time. (Latin word for time is tempus.) I think temporal shift, then means a shift in time, or a disruption of time of some sort.