Follow-up to this post:
Charlie's owners came and picked him up this morning. Apparently they're traveling in an RV at the moment, and he just spooked and got away from them, and somehow they didn't get my messages until late last night. They were VERY worried about him, though—I got out of the shower this morning to find four desperate text messages and two voicemails.
They drove right over to pick him up, and he was completely overjoyed to see them—it was very sweet (which made me very happy—I'm not sure what I would have done if he seemed nervous or afraid). Apparently he's on an anti-anxiety medication, which is why he's been so upset and clingy—he wasn't just missing his humans, he was also off his stabilizer. Even with that, though, he was doing a LOT better yesterday. He stayed home with Buster all day with no problem, and he did not pee or poop inside, for the first time since I brought him home on Saturday—he waited until I came home and took him out.
So I feel a lot better about handing him over—seems like his behavior wasn't so much because of lack of socialization or poor care, but just having his world upended all at once and being completely freaked out. And his owners were very nice, and absolutely thrilled to see him again.
I'm definitely relieved, and I'm glad he's back with his family, but I'm also really sad. I was already thinking of him as mine! OTOH, Buster is going to be crazy happy to have me to himself again, so I guess it all works out.
Thanks so much for all the advice and kind words from everybody—I really appreciate it.