I also just read in another thread (Boosting With a Linked Account) that having the 70 doesn't break the bonus, it just doesn't create it. So if main->a and a->b, the a->b link is still valid and both will get the exp.

Although if account a is a dumping ground and won't be part of the team, then none of the a->b a->c a->d a->e links will be valid and nobody will get the exp bonus ;).