Well, I don’t know what the “official” answer is, but I would say bow to the altar. Even if one were to “bow to the bishop” it would really be a bow to the office of the bishop, not the person.
That distinction made, this is what the CCC says about the altar.
1383 The altar, around which the Church is gathered in the celebration of the Eucharist, represents the two aspects of the same mystery: the altar of the sacrifice and the table of the Lord. This is all the more so since the Christian altar is the symbol of Christ himself, present in the midst of the assembly of his faithful, both as the victim offered for our reconciliation and as food from heaven who is giving himself to us. "For what is the altar of Christ if not the image of the Body of Christ?"214 asks St. Ambrose. He says elsewhere, “The altar represents the body [of Christ] and the Body of Christ is on the altar.”
So, the bishop being a successor of the apostles does not, in my book anyway, supercede the importance of what the altar represents.
As far as reference, “His Excellancy” when referencing indirectly or “Your Excellency” when addressing directly. Should he tell you personally to address him otherwise than you should do so, but only in private.
Again, these are just my opinions.