You are welcome, Pintree. As to "expert", there is conclusive evidence that dogs form bonds with their human "pack" and other animals in the household and that being moved around and/or changing owners is not in their best interests; of course, when an owner falls ill and can no longer properly care for them, different story, in which case it can't be helped (and in the dog's best interests in such a case to find it a good home, which you did by the sounds of it, and in which the dog does adjust, learns to trust and form bonds again; know this, and that you did a good thing). From a rescue perspective, we also see former owners who relinquished their dogs second-guessing themselves down the road all the time (many who "dump" their dog don't ever look back, could care less; others feel guilty, second-guess. Typically, those who worry are the ones who relinquished their dog for legitimate reasons, whereas the others to put it politely did not. It's the property of the rescue and/or new owner's either way, but many rescues and new owners will provide updates for the asking, and it is good to hear you're in touch and provided with updates; also typically means these folks are nice people, further confirming in your mind I trust that you did in fact do the right and proper thing).
But you now raise a different question, based on facts which you are assuming; is it the case that the owners themselves do not want the dog, or is it the case that you are assuming, because they've other responsibilities, they would be better off if you got your dog back. See what it is I am asking here? It is not the same thing. Unless and until they tell you--specifically--that they do not want the dog, do not assume anything other than what you seemingly confirmed: they love the dog. You know, I hear parents complain about their children all the time, does not mean they wish to get rid of them, right?
And please don't assume that just because it's in an apartment, sharing its home with other animals that it's unhappy; makes no difference whether that apartment's in Tokyo, HK, Toronto, NYC or Paris. A Jack Russel which is among (if not the most) high energy dog there is can easily be content in an apartment, so long as its needs are met, Dogs don't really care, keeping in mind that wolves, fox and coyotes from which they derive live in very small tiny dens. Ever been to HK? Some of those apartments make condos in Toronto look small, and don't even get me started on the "apartments" of Paris (seriously, have you ever been? Balconies in Toronto and Montreal are bigger than what passes over there as an "apartment". Yet dogs, inclu;ding Jacks, are happy in those units nonetheless). So, unless they specifically tell you that they do not want the dog, please do not assume anything. That they took him in, have cared for him all this time, and by your own findings are good pet parents likely means the opposite, yes? It's OK. You did what needed to be done at the time, as hard as it was, in your dog's best interests. Please don't beat yourself up about it, and it is great for me to hear that you are staying in touch with the new owners. Your dog was very fortunate that you made provision for him; many owners out there don't. When you're ready, there are many dogs out there waiting to meet you. You may not be looking right now, but know that there is a dog out there somewhere looking for you. (When you are ready).
Hope this helps, and all the best.