Yes; especially if you are taking a flight to get to Monterrey.
Yes, you need a passport to cross the U.S. border. If you are traveling by land or by sea, you can use a passport card, which is similar in size to a driver's license. If you are traveling by air, you must use a traditional passport. Considering the fact that a bus will take between 27 to 45 hours to reach Cancun (from the US-Mexico border), I guess your best bet is to take a flight; to board an international flight, you will need your passport.
No. You need an ID or birth certificate in case you cross the border by land, or a passport in case you take a flight into Mexico.
If you are a resident of the US, you need a passport to travel to ANY other nation and to return to the US.
Yes, you need a passport. If you are leaving the USA and going to Mexico then you need a passport.
Yes, babies need a passport to travel to Mexico.
Yes, children need a passport to travel to Mexico.
Yes, infants need a passport to travel to Mexico.
Yes, children need a passport to fly to Mexico.
Yes, children need a passport to travel to Mexico.
Yes, minors need a passport to fly to Mexico.
Yes, minors need a passport to travel to Mexico.