When Joseph Smith died there was some uncertainty about who would be the next leader of the church. For several years the quorum of the twelve apostles governed the church until Brigham Young eventually was called to be prophet. According to Wikipedia, Brigham Young relied upon current section 104 to make the argument that the Quorum of the Twelve should lead the church in the interim. That bring up the first question-- what does D&C 104:22-36 mean?
22 Of the aMelchizedek Priesthood, three bPresiding High Priests, chosen by the body, appointed and ordained to that office, and cupheld by the confidence, faith, and prayer of the church, form a quorum of the Presidency of the Church.24 And they form a quorum, aequal in authority and power to the three presidents previously mentioned.26 And they form a quorum, equal in aauthority to that of the Twelve special witnesses or Apostles just named.
This makes sense for quorums that have priesthood keys. Priesthood "keys authorize priesthood holders to preside over and direct the Church within a jurisdiction, such as a stake, ward, or quorum." Some priesthood callings come with keys. Others do not, but use the keys of another through delegation. The President of the church has all the priesthood keys for the church, and is authorized to use them. Similarly, the Apostles have all the priesthood keys necessary to govern the church, but are not authorized to use them (unless there is no president). The seventy, however, "do not receive additional priesthood keys, but with each assignment they receive from the First Presidency or the Quorum of the Twelve Apostles, they are delegated authority to accomplish the assignment given." (Additional Keys? I didn't think they had any keys at all.)
So, what does it mean that the quorum of the seventy is equal in authority to the first presidency and quorum of the twelve? It could mean that if both the first presidency and the quorum of the twelve are dissolve, the seventy govern the church. But that can't be the case because the seventy have no keys, which are necessary to govern. The only answer I can think of is that there is no first presidency, and for some reason, the quorum of the twelve (or at least someone from the quorum with the keys) is around, but cannot govern for some reason. It actually doesn't make sense.
Someone in church on Sunday suggested that, because the seventy have no keys, if all the members of the first presidency and quorum of the twelve should die, then the stake presidents from throughout the world must gather together to have all the keys necessary to govern. That can't be right. They would have the keys of presidency for the stakes, but there are other keys, like the keys for the gathering of Israel and the sealing power. Stake presidents don't have these keys. Plus having the keys is not the same as having the authority to confer those keys on another.