All I can say after reading all that, is that it seems you two really spend a huge amount of energy worrying about and attempting to manage and control each other's other relationships, instead the one you have with each other. No wonder there are sticky issues and questions that come up when another person enters the picture - you've both zapped each other's agency and ability to choose for yourselves.
It does seem like you're starting to see that, so keep looking. What's that about? Can you let go of the reins, and trust each other to run your own lives and manage your own relationships? The only people who should have a say about a relationship are the people in it, not anyone else on the periphery. Can you be brave enough to just be with whatever shows up and handle it without setting down rules and expectations?