I don't think any of the internal politics hold up to scrutiny at all. I wonder how messy it'll get, also how many people are aware of how broken Rei is? Why would any other nation let them have the capability to summon such beings without sharing it? For 400 years mind.
From Tsutsuji’s story, Rei is probably just run off the mill summoned hero back then. Her power is similar as Nakatani (or how their world works), making her stronger the more opponent she defeated, and she ends up being a monster like that.
And the empire probably only have a handful of people knowing about the cloning. It was said they have many factions. So even if they have cloning tech, it’s treated like their hidden stash of nukes.
As for summoning, they also have said the empire already have powerful military power. Summoning hero is actually just for a show to the public and other countries, and they thought they’re smart enough to manipulate the gullible kids. At least until things went out of their control.