So back to the question:
1) What are the requirements for a valid conclave;
2) To what degree can necessity dispense from these?
3) Who would and would not be happy with such a solution?
I don't think anyone can answer your questions with any authority, finality, or security. If they could, you could take the answers and act on them -- and you, Sean Johnson (and the priests/bishops you work with) would solve the Crisis in the Church. Deo Gratias!
But as I've said many times,
this Crisis won't be undone so easily. My personal opinion is that the Crisis in the Church is akin to a supernatural mystery, that only God can personally untangle or solve. As evidence for my opinion: I present to you the last 52 years of Crisis. We've had holy priests/bishops, learned priests/bishops, both in and out of the Roman authority structure, and NO ONE has been able to slay the dragon. I conclude it is because said dragon is not slayable by a man. And I think that's a pretty logical conclusion, if I do say so myself.
See my thread about the "One Ring". There is no "One Ring" in Tradition, before whose power all are forced to obey OR fall into malice/bad will. No, we just have a bunch of MINOR "rings of power" -- the Ring of Lefebvre whose "power" (let's say the virtues and reputation of the man +Lefebvre: personal charm, truth, logic, prudence, etc.) built up the SSPX. The Ring of Simplicity which spawned many Sede groups. The Ring of Peter which attracts many souls to the FSSP and Indult groups. And so on.
What we need is the ONE RING which will attract every Catholic of Good Will and compel them to join up. A Ring that will answer ALL the questions and doubts. So far that Ring has proven to be elusive, if it even exists yet. Only God can bring said Ring to the table.
52 years of evidence is a lot of evidence. Many things have been tried during that time. Dare I say *everything* has been tried during that time?
And as +Williamson points out, solutions like +Lefebvre (a well known, holy, orthodox bishop who established himself before the Crisis, holding Truth and Authority together by force of his personality) is no longer possible -- those who lived and worked in the age of +Lefebvre are all dead. By that I mean "those who served the Church with honor and distinction in the 1950's". Therefore all the bishops and Trad personalities alive today were at least THEOLOGICALLY TRAINED and did all their PRIESTLY WORK
after the Crisis, even if they weren't physically born after 1970.
A man born in 1970 would be 52 years old today. Not exactly a wet-behind-the-ears spring chicken! But that is nothing; what really matters are your first memories, after 5 years old. So a man born in 1965 would have his first memories either of the Novus Ordo or a Trad chapel somewhere. And a man born in 1965 would be 57 today -- 8 years away from RETIREMENT AGE. He would already qualify for membership in the AARP.