If we are selling business first and on a larger scale will much like Nike will that give us seniority use? Likely giving us more legal rights?
Only if you end up getting there. It is not seniority exactly, is greater good will and greater secondary meaning and use of the brand name.
Does it make any difference that we are registered vs. her just being trademarked?
Not really, not in terms of secondary meaning rights. They are still there first.
Does it make a difference that ours is pronounced as it's spelled, as apposed to her two words?
Absolutely not. It is still substantially similar. If someone opens a "Bestbuy" electronics, the fact it is one name will not make it be any less of an infringement against "Best Buy".