Very interesting nsw.Nice Theory you did some good job collaborating the pieces information nicely
I think that the Byakugan is a genetic trait of whatever species Kaguya is. Since it CLEARlY doesn't come from the juubi
I''v always suspected that the the Hyuga never once observed using ninjutsu. Ever. Not once. which ties in with the Moon People and Byakugan is the celestial dojutsu concludes that the Byakugan was among the first dojutsu.
If you look at the Juubi carefully, it shows both the Rinnegan and Sharingan in its eye, yet there is no Byakugan mixed in there, implying it is unrelated. If kishi wanted, he could have made juubi's eyelike obito's war mask so to represent all three eyes; tomoes with ripples and a white colour, yet he makes the juubi's eye red instead.
Then there's the fact that sharingan (in MS/EMS state) and rinnegan have demonic powers unlike Byakugan. There's also the fact that Byakugan is something hyugas are naturally born with, while sharingan has to have special chakra sprout in order to awaken while rennigan needs ashura+indra to awaken.
Also, while Rennigan and sharingan can't be deactivated by foreign users, Byakugan can be. It doesn't really mean much, but it once again shows a difference between the byakugan and the other two eyes.
So knowing that Byakugan is almost certainly NOT apart of the juubi, is it really satisfactory to say Kaguya had it and leave it at that? I don't think so, hence why I believe it is an eye that is naturally found in kaguya's species.
I think rennigan, sharingan, and ALL elemental kekkai genkais came from the juubi. I think the only two kekkai genkai that come from Kaguya and/or her species are the Byakugan and kimimaru's bone manipulation
I also believe that it is a genetic trait for the same reasons that you have stated. There is no link between the byakugan and the shinju/jubi. It is an interesting and possible idea that the byakugan is a celestial justu and is connected to moon people maybe (haha idk but I like the idea).
I agree with everything you said however, I maybe forgetting something, but could you give an example of byakugan being deactivated by a foreign user?