I will speak to the friendship that is expected to develop between you and the potential metamour.
The relationship you develop (or don't) with new lady is in the control of two people: you and new lady. This means, if she wants to be friends with you, she will need to wait for it to happen. Similarly, if you want to be friends with her, you may need to give of yourself in a new way.
I see this relationship as independent of the relationship NL and Maca share. I think communicating to this lady that you build friendships slowly, so that she doesn't think you're giving her the cold shoulder is a good step.
I don't see how anyone would have a right to demand that you two be best buds and move as quickly as she and Maca may or may not do. As long as your relationship is progressing, and you're both happy with it, then that should be enough for the other parties involved.