regarding AM first: they are considered fill-in, so can originate as many commercials s they want.
for HD2 - it goes back to whether the translator is a fill-in, or if the service contour of the translator extends beyond the service contour of the primary/HD2. (if it extends beyond the service contour, then it is not a fill-in, and thus can not take money from the primary. It is limited to 30 seconds per hour. however, if it is a fill-in, which is much of what is being done in launching new formats, then there really isn't a limitation as all commercials can originate on the HD2, which gets rebroadcast on the translator 100%.