1) Check with your credit card on insurance coverage. You may be completely covered in Mexico. If so, get them to provide you proof so you can turn down their overpriced (and in some cases unnecessary) insurance -- despite their scare tactics
2) Do a much more thorough walkaround of the car when you pick it up. Note small nicks and scratches. Take advantage of your digital camera to document condition.
3) Don't count on getting any upgrades

Roads in the Yucatan are in good condition -- the highway to Merida is in great shape (high tolls but worth it) and the highway south to Playa and Tulum is just like driving in the US other than the speedbumps. Have fun!