Says who, exactly? I mean, yes, the Imperium can't clone them - but the Imperium can't backup and restore people. And anyway, I thought that it was only very long range navigation that required psykers to navigate (see also, the Tau, who as far as I know aren't psychic).
Cloning Psykers tends to result in mutations that require killing said clones as a mercy and a way to prevent warp predators/daemons from popping in and fucking shit up. Even with a calm(er?) Immaterium, AdMech/Imperial history would show that the results of cloning psykers is best responded to by killing anyone insane enough to even try, before then burning the place such things happened at down for good measure. And this response is still the calm and measured reaction here for such situations.
That said, Psykers are likely the easiest group to put into fully artificial bodies without most 'posthuman' antics EP usually has to deal with, as the nature of the Psyker soul makes most of the EP-related post-human behaviors less likely to occur. Not that (most of) the AdMech would be insane enough to try while playing around with Psykers. :shrugs:
You're not giving me a good sense of scale here. Jupiter is big, that could be trillions of people, or just a billion, or whatever. I'm not getting a sense of the amount of habitats or industry we're talking here. Or like...who we're talking about. The AdMech is there but like...is there an effective heirarchy that's in charge? Are the Space Marines who are gonna go try and reconquer earth or whatever? Are we talking pre-Heresy? Post-Heresy? For extra fun, mid-heresy?
Like there's no single big major interactions that I can call out other than waving my arms wildly and saying "FTL exists, this ruins everything for everyone" because there's nothing specific you're saying about 30k Jupiter (just giving EP FTL, no other changes, would cause it to blow up somehow).
Even before the Great Crusade, there were hundreds of billions of individuals (at a minimum) living within the Solar System due to how built up Sol had become (most likely figures were in the single-digit trillions range), and while the great collapse in the Wh30K-verse (before the Emperor started rallying people and getting things rebuilt) did lead to a large chunk of the population dying, the population figures were still in the ranges associated with human hiveworlds. In addition to that, most Imperial ships have population figures in the high-hundreds-of-thousands to millions range per ship, and those tend to be the small ships at that.
So an easy billion individuals is believable as a lowball estimate, and that's not counting the servitors. Numbers are open to being in the ten-to-twenty billion range given all the manufacturing that was taking place on Jupiter due to the easy-radiation-as-a-powersource thing it had going for it, along with the AdMech using human labor everywhere they possibly could.
As for timing, pre-Heresy, before the first Primarch was found.
The big blank atm is 1. most of the factions reactions, 2. how they try and make contact to figure out wtf happened, 3. how they act once they realize something insane just happened and they start getting details regarding 'what', 4. how some of the EP groups try to hinder the new Jovians that somehow are worse than the old-ones, 5. how they try and use this situation to benefit themselves, 6. the newcomers glassing Terra and then terra-forming it, 6. the AdMech remnants wanting Mars in order to rebuild, 7. how the EP factions and newcomers work towards dealing with the issue of food, 8. both sides trying to figure out wtf happened and HOW it happened, and 9. the Void Clans of Jupiter wanting to make use of their ships/shipyards to grab as much power in this situation as they can.
