There was no priest for Adam and Eve, but they were able to have children. And their children were allowed to commit incest so there is a precedent for the rules not applying in extreme circumstances when it comes to it.
God also doesn’t expect us to do the impossible so if they were the last two people on Earth, I think the impossibility of being able to find a priest would allow it.
I’m not a priest or theologian so this doesn’t come from high authority, but it’s my opinion.
“Natural marriage” (non-sacramental) exists outside of the members of the Church, which is when a man and woman commit to each other in a monogamous union, and such sexual relations are not seen as evil by the CC. For Catholics, a priest or deacon are the ordinary witnesses of matrimony, but like with baptism, it is not strictly necessary in extraordinary circumstances.
Apostolic succession will be transmitted until the end of the time, so in an apocalyptic scenario where most everybody dies and the Vatican is atomized, there will still be a bishop somewhere to ordain other bishops & call men to the priestly vocation. Without a priesthood there is no Eucharist and there is no Mass, and there will always be a Mass. If there were no bishops left, it wouldn’t be possible for there to be a new bishop (unless Christ performed a miracle and ordained a person himself).
Obviously marriage existed before the Church, so it would exist “after” it.
I think post # 4 nailed it, however; the Church as a being would never be destroyed, because the “gates of Hades” would have swallowed it if it were; therefore God will never permit the Hierarchy as a whole to pass out of existence. Some places may go for centuries without Mass or liturgies, but as long as there is a human world, the Church would go on.