Very nice itinerary. Unfortunately, I can't help you with suggestions regarding accommodations for the one night in Cancun. Hopefully, one of the other Ziners can jump in on that. But on your question regarding the rental car, I would

avoid going back to Cancun and keep the car. In fact, on the day you drive from Mérida to Tulum you could easily stop by Cobá en route (and the Spider Monkey Preserve at Playa Laguna).

Plus, during your stay on the coast having the car would afford you the opportunity to explore south of Tulum in the S'ian Kan National Park.

Also, on your day in Valladolid be sure and visit the Dzitnup cenote just outside of town.

John in San Diego