Always.
No, you cannot return from Mexico to the United States without a passport in 2016.
Yes.
No. All you need is your passport.
You only need a valid passport.
It you are an American citizen, you don't need any document besides your passport, which is actually needed for your return into the United States. If you are not American and are reaching Mexico through the United States, you would need the American visa to enter Mexico; then you will have to request a Mexican visa that can be provided on the spot by the immigration agents.
Yes. You always have to have a passport that's valid past the return date when leaving the US, since you can't enter the US without a valid passport.
None other than the passport.
Yes, as long as he/she has a valid passport and can corroborate reason of visit, staying time and the means to return to Mexico (i.e. a valid reservation and return ticket).
The US now requires a passport, passport card, or enhanced drivers' license to reenter the US from Mexico or Canada. Persons crossing into those countries will be checked for those documents to ensure they have the necessary documentation for return.
Only if you return by land; if you take a flight you must have your passport with you.
If you are flying you need a passport. If you are driving over the border you can get a passport card, which is cheaper. If you are only going over the border for a day then I am not sure what you need.
If you are a resident of the US, you need a passport to travel to ANY other nation and to return to the US.