Can't remember why, but I always leaned towards Dreamland_Kris' idea of them coming to the Earth in search of a more favourable environment compared to their own planet. And therefore to keep their race alive they fuse with the humans. Was Megumi at some point implying that the borg clan needed help? I really can't remember as its been quite awhile since I read the manga.
Then after the borg clan discovered the Earth, perhaps due to tracking, the other clans tagged along and began trying to muscle in on this seemingly ideal breeding/fusing ground. They could all be different races from the same planet that is dying, crowded or simply become uncomfortable for inhabiting (e.g. severe climate changes or one species becoming too dominant and killing all others or whatever else).
I reckon the borg clan is doing it all for their own purposes of survival and the other clans are trying to do the same by stealing their idea. Although the borg clan seems to be the most organised and lighter of the bunch that we saw, since they appear to try and co-inhabit with their 'victim' without being too brutal/forceful... the Yellow Knife clan too also seems to try and be of benefit to their fused-with person. But they are still all manipulative and mostly doing it against the people of Earth's will so they are just the better of a few evils unfortunately. The humans are caught up in the tug-o-war for who gets the good spot in the universe for moving to.
That was just my impression though
All your ideas are very interesting and good.
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