I am not a secondary, but I will say what Bud said in this situation. My fiance and I were on horrible terms not too long ago. I wasn't sure if we were going to make it. Bud (boyfriend) was going to stay with me if that were the case, but he didn't think that he could do poly again 1) because he didn't think that he could do this with someone new and 2) because of the destructive toll that it was taking on all of us.
He's never worried about what others think about anything that was more my thing. But he did worry about if he was the problem which he wasn't. It's why I told you in the other thread that their relationship is not your responsibility. If I gather correctly Redpepper views you as a life partner which to me means that your relationship with her has it's own legs to stand on and while you may be scared of the new poly opportunities this change could bring about your actual relationship with her should sustain itself with or without PN.
So you should not feel guilty because you have loved and supported their relationship wonderfully. If it were to fail it's because they couldn't make it work. You can be sad for the loss, but don't feel guilty.
As for the fear of her desires/needs. That will be a whole new experience and one that I suspect will bring alot of turbulance and emotions that will need to be addressed as you go and as they pop up. The unknown is always scary, but the one thing about poly we all know is that change and the personal growth from it is never fun, but worth it in the end no matter the outcome.
Not sure if any of this is helpful or not. Have you asked Redpepper any what ifs about any of this?