r/polyamory would be a good avenue to cross-post in if you're seeking more insight :)

You are absolutely worthy of happiness, lady. There's nothing wrong with exploring polyamory, with sharing love amongst knowledgeable and consenting adults. There's no need to shame yourself or feel shame.

This sounds like you and f2 need to have paternity testing to settle once and for all who conceived with whom. I'd imagine the situations to be murky until that irons out with certainty.

Have you ever heard of the concept of compersion? It's experiencing joy because others you care about are joyful. It might be helpful to reframe that your best friend is bringing your former husband joy in a time of duress for him. That's special and powerful even though it may feel complex. Her former partner is bringing you great joy and that's something for you be grateful for, coming from a marriage that was no longer joyful. Regardless of whether or not your best friend and her former partner conceived with one another or if she did so with your former husband, it's beautiful that they're finally having a shot at parenthood after for yearning and desiring and trying for so long to achieve this. Whether that be separately or together, it sounds like you and your husband coming into their lives has brought forth some exquisite change.

Blessings to all of you. Keep your chin up. Just keep communicating- openly and honestly with one another. You guys got this.