Renting a Car at Del Bajio Airport

Your previous advisor is doing you a favour.  Please follow that advice.

     You will be much better off taking any of the shuttles or even a taxi from the International Airport at Silao.  Once in San Miguel de Allende you will note that there are avalanches of taxis....cheap to relatively cheap, clean, and driven by non-madmen.
     You would even be well advised to take a taxi to Guanajuato City, say if you find a taxi driver willing to drive you there and back. You might, during these times, be able to go and come during a whole day of poking around for 1,000 pesos ( 40 or 50 dollars....but use pesos).  There are all kinds and natures of very short day or morning ''poking-around'' jaunts for which you can use the taxis.
     You will be impressed with the cleanliness of the taxis as well. And you will be able to understand many of the drivers, while learning a good form of Spanish, although many of the drivers speak some to considerable English.   You will also be able to do a lot of "free" driving around by taxi for the money you avoided paying the freight on what Mexican rental liability and collision insurance costs (two or three hundred dollars for a week).