Good to know. I had always just assumed, based upon a facile interpretation of the matter (as well as assuming I knew more about the matter than I did, wasn't the first time, won't be the last), that three bishops were used to ensure that there could be no question about the apostolic succession, no question about the validity of the consecration, a kind of redundant "quality control", if you will.
I reasoned that it was the same as what I do when I go on a trip, and check the oven and rangetop burners twice, to make sure they're not on. That's not OCD, that's wanting not to return to a BDH (burned-down house).
I think in normal times it was still required to have three bishops some of reasons you mentioned, and no one could abstain from doing so without a dispensation. Yet, God allows extreme measures for extreme situations.
It was also probably easier to have fill-ins from priest for the mere ceremonial aspect of things to remain the same even though they bestow no power than to have no one stand in and there be confusion of what to do.
A friend was telling me last night that apparently there is a thing called "stick subdeacons" where you can still have a solemn high Mass without a subdeacon and a seminarian taking his place if one is not available. The "stick sub-deacon" would do everything a subdeacon would do but just abstain from certain things that only a subdeacon had the order/power to do.
Glad I asked here about all of this! Thanks everyone!