I am really sorry you are having such a rough time. I can emphasize as my situation is kind of similar, even though I am the healthy one so far and my men have to deal with all this limited fertility and infertility shit. We have had our first IUI and I thought about sharing what my doctor told me. He said that the chances to get pregnant with this method is nearly as high as having conventional sex. It can help, when your cerviacal mucus is aggressiv towards sperm but overall the advantage of an IUI is about 5% higher than 'normal' sex.
Having covered this point I would like to tell you about my experience with suggesting another man could be the biological father to my husband. He wasn't thrilled at all. Especially as he had received the news that he himself couldn't be a father easily. And this other man in question is my other partner (who turned out to be infertile but that is another story). It's an enormous stressor for men to handle the thought of another father your children. Especially if they don't like them. So I would think twice about suggesting this to your husband in this situation.
And in regard to your thought about having a baby with this long time friend of yours: friendship is not a romantic relationship. Are you sure you want him to be the father of your child? I wouldn't go down that road as long as I wasn't sure of the qualities of said person as a partner and a potential parent. You don't really know how well you two mesh up to now if I got that right.
Wait some more and concentrating on your treatment seems to be a good way to proceed right now. It doesn't sound like your ground for poly is set yet. Your husband seems insecure, stressed and not totally open to that idea and maybe he is as occupied by the baby-topic as you are. Don't stress your relationship even more by adding poly to the mix.
Wishing you luck and success on your way.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.