...or maybe you should just hold tight and let the current emotional turmoil settle before contemplating causing anymore.
It is a very long post and but I think the crux of it is that your boyfriend is now in a different place (ie separated) so he has to try and find his way as a mono in a poly relationship, rather than a poly in a poly relationship. They are two very different identities.
To maintain your relationships you are going to have to be really, really stable in your position so that both the men in your life can feel stable in theirs. This is obviously also bringing up your husband's insecurities now that your boyfriend is available, hence original fight. Fights are not always about what they seem to be about at first glance.
I think everyone involved will be feeling sad right now and your way through is to dig very deep and mine for large quantities of empathy and communication.