Just wanted to throw out a third option beyond P primary and C secondary versus C primary and P secondary... P and C co-primaries, which it sounds like you are acknowledging will to some degree need to be the case anyway if, in fact, C ends up taking on a more primary-type role. This, of course, would be predicated on her being willing not just to meet you but to embrace you as, basically, a sister-spouse. May or may not be a workable option, but it seems to me like it could work just as well as either of the other two options.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.