There is no paper work needed to be filled out anymore to get through immigration. once you get to the immigration officer they ask you how many days you will be staying in Mexico to put into your passport and in their system. We highly recommend asking for a week longer then you will be staying. There is no down side to this and if you have any issues or change of plans you are covered from having to pay a fine if you overstay
Use Google Maps I think Waze works as well. Apple maps will get you lost.
Yes if you want a better exchange rate. you'll need your passport with you to get money at banks in San Miguel and ATM machines work but you'll get charged a fee by your bank. BoA charged me $8 for the exchange. If you have the time I would suggest calling your bank and asking them if they exchange Dollars/Pounds for Pesos and if not where you could exchange it