And maybe the Calabim need that huge production bonus because the Kuriorates or the Amurites are very powerful. Unbalanced is unbalanced in comparison to the best competition.
What?
Calabim are separately already very powerful, thanks to their Vampires. Copious XP and access to some spell casting promotions? That's really strong... overlaps with the Grigori's gig, except that Calabim don't stop at 100 XP, and can keep building new ones rather than waiting for them to pop. That's their distinctiveness and their power. Having a normal building which is basically the same as one of the most powerful wonders in the game is just insanity, and completely unnecessary.
You're right that a clear argument can't be made why 1/2 is ideal, but what form would such an argument take anyway? Do you want a spreadsheet and a thorough statistical analysis backed up by simulations and computations? The point is that 1 production per 1 unhappiness is way too much... in my opinion, it should probably be a lot lower than even 1/2, but 1/2 at least is a lot better than 1/1. Taking "way too much" and then making it some other arbitrary lower number is perfectly straightforward.
If you don't understand why the building is unbalanced, compare it to other buildings that give + something. Elder Council, for instance, gives +2 research; Market gives +3 Gold/-1 research for a net of +2. Some things give copious cultural boost, but excluding culture, giving a net of +2 to one or various things is fairly standard. Some of the buildings are unusually strong: gambling house, for instance, gives +3 gold and usually several happiness as well, and so I'd describe the gambling house as probably the single strongest normal building. As Governor's Manor is a UB, and I'm all about civs having powerfully distinct things, buildings and otherwise, I think we could use this as a reasonable standard, that the benefit of the Governor's Manor should be approximately in line with the Gambling House, rather than with the Elder Council.
What does it currently give? Often upwards of 20 production per city in larger cities, +10 in mid size. +20, in addition to its other effects. Compared to the Gambling House's +6, which I've suggested is at least among the strongest, +20 is insane... even though I believe it's okay for a UB to be unusually powerful, having it more than three times as powerful as the best normal building, in addition to also having other benefits, is just simply too much. Dividing it by half would still make it exceptionally strong; +10 production in the largest cities, making it still the single best normal building in the game (but no longer by a factor of three), and +5 in medium size cities, making it still damn good but not the absolute best (some would argue of course that production is worth more than gold, and so that it's still the best).
Does that make any kind of sense to you? I really have no idea why you need this spelled out, it seems perfectly obvious to everyone else.