Hi, I'm sorry you're dealing with MFI, sometimes it can be really overwelming.
I don't know what the natural pregnancy rates are for all the different kinds and degrees of MFI. But, while trying to keep it natural, I never missed the chance of BD when I knew I was ovulating. Except in some particular cases of severe MFI, where you really can't hope for a miracle, nobody can give you a 0% chance of ever getting pregnant (if there's nothing wrong with you, that is). I know I'm probably more likely to be hit by lightning than by getting preg on my own, but I won't let that stop me from hoping.
I don't know if you ovulate regularly or not, but I just looked for EWCM during natural cycles. I stopped using OPKs a while ago, because I'm more or less regular and get O symptoms.
Good luck! I hope you get some answers!