Current implementation of the NorsuTemplateAssignment.isAvailable method catches the AbstractMethodError and if so it returns false.

Check if the AbstractMethodError is really needed in this place.

NorsuTemplateAssignment is only instantiated in the pages-app (norsu variant), otherwise the default implementation (info.magnolia.rendering.template.assignment.MetaDataBasedTemplateDefinitionAssignment) is being used.

TemplateAvailability is widely used (not only site module) , see attachment.
SiteAwareTemplateAvailability is a singleton, see if this can cause trouble since we need to support both the jcr and the norsu at the same time