It's seems like a stretch to try to cover Larian's poor writing and implementation, I'm afraid...
The relic is gith in design, and if it were in the control of the mindflayer that grabbed us, knowingly, and that mindflayer had access to several individuals it could use against us as plants, and the relic was destined for Baldur's Gate.... this it would far more easily have just been able to drop on such individual off with the device when it grabbed us from there. The idea of giving the relic to a test subject and abandoning them in the wilds, to get them to bring it back to a place you already were, is loopy.
Devils do not require any kind of invitation to enter a home or location, that's Vampires, and only really vampires.
If the others are various plants designed to guide, railroad or trick us into doing certain things... why? What sense can that possibly make, if they're all dopplegangers, and we are as well, and we're all made and sent by the same entity - we're literally on the same team, there is zero reason to keep that from any of us or to use some against the others to guide them when they are a like-made cell of individuals.
The fallen paladins are getting their juice from Zariel, and you can confirm this with some speak with dead.
Others can look into our MC's mind, and indeed they seem to have better control over doing so than we ever do. They automatically know everything we say or do, and what choices we make, or tell others, as well as the contents of our dreams.. but we never know what they are thinking or doing, or the contents of their dreams. They are able to lie and deceive us at numerous turns, and we are not able to see through this - we aren't even presented with normal insight checks to doubt or mistrust them, and are simply treated like a fall-person for the sake of their deceptions or self-aggrandisement.
Lastly, why dopplegangers, from the illithid perspective? Why that at all - what do they gain, in their plans, from making dopplegangers, and where did they get these dopplegangers from - they are sentient, sapient creatures and a living species of people unto themselves after all, not grown tools, like intellect devourers are. What is the purpose in a) capturing a group of dopplegangers, b) coercing or breaking them into servitude, c) capturing another set of individuals, d) getting your dopplegangers to mimic those new people, e) sending them on a mission without telling them anything about it, and getting them to imitate those various random people you pulled off the street while doing so.... instead of a) capturing a group of people, b) breaking or coercing them, c) sending them on the mission as themselves, able to blend back into their old lives as necessary to do the job. I'm not sure what adding dopplegangers into this equation buys them.
