I did this in November with a group of 26 (ORD-MEX-ZIH). We were on UA819 from ORD and parked at the Int'l terminal. 4 people in our party required wheelchairs, so we just followed them to the tranisto area. When then went through customs, had our pasports stamped, and walked back through the same customs line and back into the terminal. This process took under 30 minutes from gate to gate.

Regarding checking in, half of our party got boarding passes for the connecting flight at ORD and the other half didn't. All our bags were checked through, however. There is a Mexicana service desk next to the transito area in the event you don't get your boarding passes at ORD.

Give yourself PLENTY of time returning back to the US. We had 2 hours and barely made our flight. Our flight from ZIH landed ontime, but parked in the furthest section of the airport. We used an IAD-type transport to get to the terminal and then had to walk at least 1 mile through the airport to the UA desk in order to get our boarding passes to ORD. Fortunately, there was not a line at security and we made it to the UA gate 5 mins before the doors closed.

Good luck and enjoy Mexico!

-Ualisthebst