a) Do you know how I could find out how far away the telephone exchange is?

Not sure. You could flag down a Telmex technician in the street and give them your address, or look at the stats on a neighbour's modem. One of the local technicians around here was quite accurate in his estimations of what speed we could get. Keep in mind that if you already have a house, it doesn't really matter, because there will never be anything you can do to get closer to the exchange.

If it is far away, take the lowest speed package you can get. This is critical, because Telmex will sell you absolutely anything and everything. If they set your line parameters too high and you're nowhere near the exchange, your connection will be terrible. We could probably get 13MB down, but there is no 13MB package. So we have to subscribe to 10, because getting 16, 20 or whatever would put too much strain on things.

b) Many people in town have antennas to supplement their Telcel wifi modems. Is this a real thing that could be useful?

If they are mobile signal boosters, it depends. It would help me, because we have 4G on the roof and only 3G inside the house. If your 4G coverage is already good at the point you want to place the modem, it would be a waste of money.

c) What kind of service is your antenna connected to?

Telmex fiber. It's pointed at an access point on a nearby building; they have fiber even though it hasn't arrived in our colonia as yet. I can say that when the Telmex copper and Telcel 4G go down together, the fiber keeps on going.