I notice Hyatt Residence Club have different categories for their units (upper, mid, low), but Redweek told me that is only guaranteed for owners. So even if the owner's timeshare is an upper floor, the guests can potentially get a ground level unit? I am thinking to book a three-bedroom unit, since there are only 12 total, and owners get higher priority, I am really worried about the chance of getting a dark ground floor unit.

There are owners at locations other than this one, that exchange their week for one here, in order to rent it out. That is a club exchange and technically violates the rules. With a club exchange, their is no guarantee of a room band (upper, mid, low). If you rent from a legit owner of this resort, you are guaranteed the floor band. When you get the confirmation from the owner, what you are looking for is HRPP in the reservation details (indicates a Home Resort Preference Period reservation, which can only be made by owners of this resort).