A big THANK YOU to everyone who responded with suggestions! (there were also about 25 more suggestions on my fb page...) It was interesting: all of it was helpful, even if I disagreed with it, because it helped me frame what I believe would work for us.
Here's what's happened: I unplugged the lamp in Clark's room (he has no overhead) and told him it was broken, and then I hung up a pretty little string of multi-colored japanese lantern lights. When I brought him in the room to show him the lights he said, "So beautiful!" That night in bed he stared up at them for ages and was so entranced with them that he let me leave before he was asleep.
The next night we had a harder time when Mitch told Clark he was leaving the room. There was screaming. Mitch left anyway and Clark screamed for a while more before he climbed out of the bed and came downstairs. I took him back up and sang him a song and told him I was going to leave. He protested, as expected. I got the bear off his changing table and put it in his crib, and told him the bear was very sleepy and he wanted to be sung a song so he could go to sleep. I asked Clark if he would sing the bear a song, asked what song he thought was the bear's favorite. Clark thought Row Row Row Your Boat might be, so I suggested he sing it to the very sleepy bear, and then I left. From downstairs the monitor told us that he sang and sang to the bear, and then happily talked to himself until he was asleep.
The next night Mitch was the one putting him down again, and when he told Clark he was leaving Clark screamed, but only for a moment. Maybe we've turned a corner. Aaaand in the middle of the night Mitch dreamed Clark had climbed up into his arms, and then he woke, and Clark indeed was in bed with us, wrapped in Mitch's arms.
Since then we've been up and down... most nights we leave before he's completely asleep. Since we turned out the bright lights he definitely sleeps more consistently until morning, rather than getting up and 2 or 3 or 4 and wanting to get on with the day. Some nights he has a harder time than others, and we adjust; we stay with him a little while, or we make some kind of deal like we'll leave the door open and books in his bed as long as he will stay there, and some nights we just muddle through. But! Overall we're in a better place. Yay!
Frances's 4th birthday is the day after tomorrow and I'm busy now with the making of princess crowns and the creation of a castle cake, plus family is in town for the festivities, so it might be a few days before much more.... Though I do want to say that the birthday ceremony today at her Waldorf school was the sweetest thing I may have ever seen and it was all I could do not to blubber right there in the middle of it. I'll try to post pictures.