Hello, I'm trying to put together an informal key to caterpillars for my lab, and I was wondering if you might be able to direct me to a resource that covers Amphisbatidae (or other leaf tying/rolling families) identification?

Yes, this larva could be something pyraloid. Chaetotaxy would immediately tell the tale (e.g., two L setae on T1 in Pyraloidea, versus three in Gelechioidea), but getting a close enough look to see it can sometimes perturb the larva to the point where it won't rear through. It's best just to rear it.

that appear to be going into prepupa (turned pink and fat) but they are making no move to spin a cocoon. Do they need special conditions?

they made it to adulthood? I'm not convinced this is currently posted in the right family...

and I just caught another one this afternoon, but decapitated it with the container cap. I'll be trying again on the same host (Crataegus) through the rest of the summer.

I would suggest putting a little bit of tissue paper into the rearing container, in case the larvae want to wrap themselves up in something to pupate. For rearing them through pupation, if possible put the container somewhere (e.g., porch or garage) where it will get natural day length and temperature (but avoid having the container in a place where it will receive direct sunlight, as that can fatally overheat the insects). If adults haven't emerged by about a month from now, they probably are going to need to overwinter. Good luck.

will update pics if we get it to pupate/eclose.