A few smallish updates:
1) He's basically acclimated to being left at home during the day—no more barking or destroying things (though I learned the hard way that leaving pizza boxes with a few slices left on top of the garbage can to be taken out when I get home is no longer an option). He still doesn't like it—after our walk every morning he tries to go to the car, and the last thing I see as I drive away is his forlorn little face watching me out the window—but he's become quite stoic about it. And he's so funny when I get home every night. I take him for a walk first thing, and he's so excited, and has so much energy, that for the first 10 or 15 minutes he just bounces up and down and runs back and forth as far as his leash will allow him. But as soon as he's burned off some of the energy, he remembers that I was gone all day and he missed me, and we have to take a break from the walk from a few minutes so I can pet and cuddle him. Every single day. It's so damned cute.
2) He let me give him a bath! This seems like a little thing, but he was terrified of water when I first adopted him, and he's still very wary about anything larger than a puddle, so I've just been making do with pre-moistened wipes, but he really needed a full bath. He didn't enjoy it at all, and he kept giving me these betrayed "I thought you loved me! Why are you torturing me like this?" looks, but he generally bore it like a champ (he didn't even bark or whine!), and now he's all soft and silky and nice-smelling. And I showered him with cuddles and cookies afterward, so he forgave me pretty quickly.
3) He's continued to use the litter box (he's been having some diarrhea issues, so he can't always hold it when I'm not home to take him out—the vet is going to do a stool sample test this week), which is pretty damn amazing. And last weekend he started breaking up tussles between other dogs at the off-leash park. It worries me somewhat that his instinct upon hearing dogs frequently 10 times his size snarling and barking at each other is to run and put himself in the middle, but so far he's done a wonderful job of playing peacemaker. Last Sunday someone brought a large German Shepherd wearing a muzzle to the off-leash park (trying to socialize him, I think), and he and another dog promptly got into a tussle, and Buster spent the next half hour keeping the German Shepherd in his sightline in case there were any more problems—he didn't go back to playing normally until the Shepherd's owner took him home.
tl;dr I have the sweetest, smartest puppy ever, and I love him so much I don't even have words.