I'm developing on a Fujitsu 4120c2 scanner, and acquisition no longer triggers the device event. Even using the acquisition demo (unmodified), and low level twain demo, the Paper Jam event (or any device event) is not thrown. I recall it being triggered before, so why is it not firing now? Also, I can't even set the device event to paperjam, it ...

The Device.QueryCapability method should give you the information you need.  However, that depends on the driver reporting correct results. The real way to get this information in TWAIN is to use the DG_CONTROL / DAT_CAPABILITY / MSG_QUERYSUPPORT triplet.  The driver will return a bit field containing the supported triplet ...