There is something very sweet and vulnerable about your post. You seem to be trying to take it all in stride and I applaud you for your wish to understand these people in your life.
I think it's safe to say that anyone when first dealing with the concepts of poly, has a hard time rearranging their thought pattens , because it goes against what most of us have been taught.
I think most people are more comfortable with extra-marital affairs than they are with polyamory because with a secret affair, there is that ownership, to share your own universe- feel like you're the only one if even for a few hours at an opportune time.
Oh, look! I've made it about me now.
I don't have much experience but I've lived a few years, and the adjustment is just a difficult one- life-changing, in fact. A catalyst for all of your fears and social programming. Give it time. It seems you are willing to ride out the curve and teach through your actions.