More than just the "basic details" are needed to fully answer your questions.
In your OP, it says:
"because of the more than dozen places you have initialed to indicate accept/decline various options, one shows a fixed exchange rate printed there."
Could you expand on that a bit more?
What sort of "expansion" are you looking for? I donīt have the contract at hand, but I believe there were at least a dozen places to initial there at the counter. It turns out one of those places did show "1 USD = 11.85 pesos," but I donīt know that I looked at it, or if I did that it should have put me on notice I would not be able to pay with my US AmEx card in USD, especially since the total rental was stated there on the contract in USD, nowhere in pesos. (Additionally, in answer to a question about what it would cost if I were not arriving by plane and renting, the clerk told me something about how I was actually being advantaged by a more favorable exchange rate than a local would pay to rent. I paid this no attention, because it sounded like mumbo-jumbo, and I figured I would be paying the rate central reservations quoted me in USD, the same rate in USD they then confirmed and which appeared in USD on the contract print outed by the Mexican location.)
Am I alone in seeing this as something of a scam, that is seeing a charge 10%greater than the one I booked at? Others would have no problem with this, or they would have been prepared to pay in cash (USD $) when they turned the car back in?