Well STB, the 'fat lady' has sung and what a story. I think some people here forget sometimes that you are indeed a cuckold, and that Sue's activities are intensely arousing to you, if also at times nerve wracking. It's the flip side of the same coin of course.
I noticed you passed quickly over the implied serious relationship with Brad. Maybe because its history, maybe because it still creates a more worrying feeling for you than her current relationship with Don. Still, if she can go this deep once .. maybe not though. That one was her first, and had some unique elements.
I don't get the feeling that anything you are doing at the moment is threatening to your special relationship, it can certainly be reversed by you both if you want that. Sue is increasingly aware of what excites you, and what excites herself too. These two are remarkably meshed at present. Don is maybe more that just a tool to supply the missing ingredients, but not at present much more emotionally. He will maybe get frustrated that he can't get more of Sue's heart but if he's wise he'll settle for what he does get, and he seems smart enough so far.
This is such a great thread STB, my thanks once again for sharing it with us all.