In a comment in the previous post, there is some curiosity as to how the Imperial Couple is selected. So, behold, I answer:
It’s semi-hereditarian. The heir is notionally picked from among the members of the Imperial family, in an attempt to capture the hereditarian advantage of having someone trained for the job lined up, not just some random schmuck1; especially since the Imperial family also serves the Imperial Couple as a talent pool for extraordinary tasks so they can get an idea of what their on-the-job performance is like.
But it’s not directly primogenitive, etc.: the current incumbents get to nominate their heir from among all the possible candidates, so if Mr. Firstborn wants to succeed to the throne, he’s got to work hard at putting himself out in front of the rest of his generation. And also any really exceptional candidates from outside, because succession-by-adoption is also part of how the system works.
After that, first, in order to be nominated in the first place, you have to be, well, a couple. This is a diarchy; the system’s not set up to have singletons on the Dragon Throne. It would eliminate checks and stabilization factors that are supposed to be there. (You also have to be a happy, well-adjusted, non-dysfunctional one that’s capable of working together successfully, but that pretty much goes without saying.)
(Now, as for triads and other topologically-different marital forms, to broach the obvious question: well, it will be an interesting day, Charter-law-wise, when one of those is the best candidate for succession, but it hasn’t happened yet.)
After being nominated, as a check to ensure the process is working properly, they have a triple gauntlet to run:
First, the Senate can veto successions they don’t approve of, which eliminates anyone who either lacks the arete to lead – which, eldrae being eldrae, culls everyone who isn’t an adequately polymathic genius with a history of achievement in multiple fields to prove it – or who can’t garner enough support to lead.
Second, the Eupraxic Collegium can veto anyone who doesn’t meet their strictest standards of sanity and rationality, because no-one wants a crazy person on the throne, even a well-hidden one.
And third, they have to be accepted by the Imperial Presence, the composite mentality of Imperial Couples past dwelling in the Transcend’s mind, as a subset of itself.
…but after fulfilling all those hurdles, then they get to be the officially designated heirs.
1. Just to continue a little on the theme of the Democracy Is Bad trope, while I’m at it, the Imperial opinion of the sort of people we put in charge of various executive branches on Earth is that while the process does ensure that they have some talents in the areas of rhetoric, amateur memetics, and graft, their gifts in the areas of actual leadership and sovereign administration wouldn’t qualify them to run a lemonade stand in, y’know, civilized parts.