I don't understand why you can't just end it with the husband and everyone else do what they want.
This, except I do understand. Many people, couples, new to poly think they have to both "be on this journey together." They think it's NECESSARY that they both either swap spouses, as here, or that a bi woman and a hetero man both share one bi woman.
This is not true!
Couples do NOT need to share one partner or be in a quad/spouse swap configuration in order to practice polyamory successfully. In fact, most happy poly couples do NOT share partners or do quad-shape relationships. Each partner just dates their own other partner(s). This is my case. I am in a 14 year relationship. I have a bf, my female partner has a bf. We do not go on 4way dates, we do not have group sex. If she breaks up with her bf, I do not need to break up with my bf. lol. I've had a few relationships lasting X amount of time and they have no effect on what my gf does, or whom she dates.
In this case, your relationship began very badly. Yikes. Your husband cheated on you with Cherry, and Banana was informed or found out, and all 3 kept that info from you when you all supposedly swapped partners for the first time and then for 6 months thereafter! Yes, you've had therapy, and you seem to have regained trust in all 3 of them enough to continue on as a quad, rings and all, for 10 years. Yet, once again, those 3 are ganging up on you and trying to coerce you to have sex with Banana against your will, just so that Apple and Cherry can continue to bang? You all are mature adults. This is the best solution you can come up with?
I was in a 30+ year relationship/marriage, and when I was done, I didn't let anyone tell me to continue. They may have thought my husband and I were the "perfect couple," and they enjoyed seeing that. We were not perfect, we'd become unsuited.
It seems like you're dragging your ass through this crap out of some sense of loyalty. I am glad you're letting go of the idea that you need to stay "committed" for the rest of your life just so the other three (who cheated on you at the very beginning) can continue along in some sort of balanced quad.
I agree with the other posters: you should break up with Banana. It's not acrimonious. You just don't see eye-to-eye anymore. You are allowed to feel what you feel. Quads and triads rarely last 10 years. Each person has a right to love or fall out of love, to honor their own feelings. You don't have to fuck someone you're no longer attracted to just because that man still desires you, and certainly not so that Apple and Cherry can maintain a sense of false reality and balance.
Friends often share beach houses/vacations. You don't need to be lovers to share a beach house for a couple weeks.
You could seek therapy again, for yourself, or with Apple, if that would help. Please honor your own feelings. You have a right to have them. Advocate for yourself.