Hi, Henry!
Just a few questions and some remarks:
You say you have a functional code, but what exactly do you mean? Does it compile? What estimates and diagnostics are you getting when you run it? What help do you want from the community?
Looking quickly over the code, and not knowing much about your data, I don’t immediately see a problem (but I’m no expert). Is your half-normal (0,1) prior on alpha working well? I’d think it would place a lot of weight on 0 discrimination, which is unlikely, given that the items are designed to measure something, and perhaps a little tight for high discrimination?