I'm with you here. I can't imagine how risotto could be successful in a crockpot. The whole point of open-cooking it is maximum concentration of cooking liquid flavor - much the same philosophy as a paella.
To address the original question:
Yes, scallops can be substituted for shrimp, but you must be very careful to adjust your timing of adding them. Up for sea scallops (which are larger and denser than most shrimp and will require slightly longer cook time) or down for bay scallops (which are much smaller and hence will require less cooking time)
This is ESPECIALLY important if you're preparing a dish in a crockpot, regardless of what dish it is (and whether I agree with using a crockpot to prepare it or not
) because crockpot cooking times are dramatically different due to the nature of the cooking process.