If you look at swinging as a sliding scale spectrum, and poly as a sliding scale spectrum, then you can say I am more at the poly end of it, while my husband is closer to the swinging end of it. He still has interest in poly, but more based in friendship-love then romantic love.
At one time, for us, we had a relationship dynamic going on quite closely to what is going on with you and your wife, and her new bf.
My husband voiced the same concerns you have voiced here.
What ended up being the best option for us, was negotiations on both sides.
I am not opposed to the swinging lifestyle, but do find myself very much needing to know people. I am not into casual sex. It doesn`t do a damn thing for me.
On his wants : The decision has been to make good friends with people, and if a friends-with-benefits situation develops ( the true meaning of FWB, not the new booty-call version.)...to go with the flow.
On my side of things, any relationship I develop, he is in the know, and I now have the past experiences to keep the NRE in check, and keep my home life charged on full batteries
A thought crossed my mind awhile ago, that however much energy I am putting into a new relationship, I need to put the same energy (but differently) into my exsisting relationship. For me, this mean discovering something new and fun with my primary partner, that we haven`t done before. Maybe a new hobby, a new kink, a new place to go to and enjoy. Something that excites us both, that we can experience together.
This tid-bit has really kept the harmony for us.
The other thing we do, is encourage each other in the others' more 'natural' lifestyle. He has my back, and I have his. This is without question.
I haven`t read the other posts, but I wish you well !