Okay. So it's not conclusive, but I would like to update our sleep experiment. I feel successful!
The baby is ONE (oh, my!) and he is sleeping through the night consistently. I usually put him to bed between 6:30 and 7 p.m. Sometimes he wakes between 5 and 6 a.m., but after nursing he goes back to sleep until about 7 a.m. This makes me feel great!
Key #1 was putting him in his own room to sleep. We temporarily moved L out of her room so that he could be alone. Even though I generally think Americans over-value independence, I did want the boy to be able to look around, realize he was alone, and be okay with it.
Key #2 was that he was in his own room- down the hall from me. While I did keep his door cracked and mine wide open. . . I was able to sleep. And I sleep hard. It sounds callous, but if he was crying, I probably slept through some of it- allowing him to learn to go back to sleep without me. (I'm not kidding; in college I once slept through tornado alarms that had the rest of the dorm in the basement.)
Key #3 was his blue blanket. He's fallen in love with it. He sticks his fingers in his mouth and sniffs that blue blanket and all is well. (And I didn't realize how important this was. The Lawyer and M and I just had a weekend away and I failed to bring the blue blanket, assuming the green "car" blanket was sufficient (the one we keep in the car for him that is oh, so similar to the blue blanket). It wasn't. M spent most of two nights in our bed, barely consolable.)
That's all I can come up with. Nothing amazing. He's still nursing, still taking two naps each day, and still sleeping in the pack n' play. And now he's ONE! (I know, I mentioned that earlier; it's still a bit of a shock to me.) We've moved L back into her room and he seems to be doing just fine. . . and therefore, so are we.