Are guests able to use Westin Resort and Spa amenities as well?

Yes, as a Renter you are filling-in for the Owner. You won't get points, neither does he. The Owner is trying to recover some money for the maintenance fee, usually falls short. (Fell sick, money problems, all kinds of other Reasons). So be kind to the Owners don't be proud to snag a good deal. You will have a much better Vacation