Theory 1:
When on the timeline does Hitsuguya become a captain? I was under the impression that he is a relatively new captain (because he gets all the runt jobs like having to go to the real world) and also, I dont recall him being mentioned at all int he pendulum series. Could Isshin have been replaced by him?
Theory 2:
There is an accounted squad, sqaud 0. Maybe Isshin played a part not seated in any particular accounted squad, but was the captain of it?
Oh boy a thinker
you first theory witch I will hace to call theory 3 plays into one of my favorite Ishhin theorys (witch we can call theory 5)
That Isshin was Captin of Squad 10 around 20 years before the story started, on a mission to the living world he fell in love with Masaki
who either saw him in a gigia or she could see Sprits and told him he looked cool when he smoked. Isshin as a captin knows that the traitor Urahara lives nearby goes to him get specil Gigia and starts a family.
As I said this is my favorite theory, my real proplem with it is that although there is no evidance againt it there is also no evidence for it
the other theory that you posted (theory 4?)
is another one with no evadence one way or the other, but It does lead to the potentioaly cool Idea that Masaki was an allies for Kirio Hikifune