Okay, we seem to have finally found something that works for getting Gwen to sleep - whether for naptimes, bedtimes, or after-3am-feeding-times. It's not easy for any of us, but it WORKS, and incredibly? It's so simple!
We put her in her crib and walk away. She cries (and sometimes screams), and then after a while she stops, lays down, and goes to sleep.
After spending nearly two hours trying to get her back to sleep at 4:30am, during which she screamed anyway, we finally threw up our hands at went back to bed, where we lay in the dark, listening to her cry, telling each other we weren't bad parents. Twenty minutes later, she went to sleep. Those 20 minutes weren't easy, but they were marginally easier than the 2 hours beforehand where we listened to her scream and we were standing up.
I can't believe after everything we've tried that all we needed to do was get lost and let her do it herself. I can't believe that after all this time we just needed to do what we started out doing in the first place. Hello! We are first-time parents, please ridicule gently.