I for one think you are doing very well with these changes and challenges all things considered, I'm guessing personally I would've cut and run from Mono a few months ago, as the idea of a partner cheating (whether it be physically, or being open to it) is the most terrifying thing for me. However it works out for you, I think it takes strength to not jump to rash decisions, and to take the time to make the decisions that are right for you.
I don't envy the position you are in. I think it's a very interesting mental shift you've decided to try to make mentally from "family" to solo poly - one that doesn't have to mean anything in practice, but obviously can have both pros and cons emotionally. I'll keep my fingers crossed that it's mostly pros for you.
Happiness will never come to those who fail to appreciate what they already have.