The offering Jesus did was "once for all". The previous offerings were only temporary until the real one was to be done. The bible says He remains a priest continually, but does it ever say that He remains an offering continually?
Already discussed. Hebrews is drawing a contrast between the sacrifices of the Old Law, which were repeated sacrifices most of which applied only to one individual, and Christ's sacrifice, which is one single offering for all mankind.
By the way, ever noticed Hebrews 13:10?
"We have an altar from which those who serve the tabernacle have no right to eat."
Wait ... "we," the Christians, have an altar, which is, by definition, a table for sacrifice, from which those who serve the [Jewish] tabernacle have no right to eat, obviously implying that Christians do have a right to eat from that altar.
Protestant commentaries fumblingly explain away that verse, saying that "altar" really means "cross" and "eat" is, uh, metaphor for ... um, er, "believe" ... yeah, that's it.
Catholic commentaries, OTOH, don't have to.