I think, in the end, even this approach will require God's intervention. For all of the Traditional clergy to agree would be a miracle in and of itself.
Agreed. Not only do we have the division between sedevacantists and non-sedevacantists, but there are many division AMONG each of those groups (neo-SSPX vs. Resistance in R&R, and divisions among the sedevacantists, such as sedevacantism vs. sedeprivationism, dogmatic sedevacantism and moderate sedevacantism, etc.). So if we can't have agreement even WITHIN these two camps (SV and R&R), how would there ever be a universal consensus across all the different groups? God will have to provide some dramatic intervention for that to happen.
So, for instance, if a group of straight SVs were to hold a conclave, not only would the non-sedevacantists not accept the "election," but the sedeprivationists would not either. Then, if the current "Cardinals" somehow miraculously elected a +Vigano, the straight sedevacantists would have issues with that, as they don't hold the current "Cardinals" to be legitimate in any way. No, God will have to solve this mess ... and He will ... and His solution will far exceed anything we could come up with.