OK, fair enough.

The 14 hour layover at MEX is part of the problem. The reason that it's showing separate baggage charges for the UA legs and the NH legs is that, per UA policy, you will need to pick up and re-check your luggage at MEX: Overnight, long layover/connection check through baggage question (consolidated)

As for how much free luggage you get -- that's really difficult to say. I think it will be 2 bags at 50 lbs each, because I think it's going to be figured based on a US-China trip (assuming your final destination is in China), And, since that's what's on your receipt, I think you can argue for that successfully (as opposed to the US-Mexico allowance, which is zero). Since the DOT requires that the charges be correct on the receipt, if they do charge you, and refuse to budge when you argue, file a refund request and a DOT complaint after the fact.

There's a possibility that you might get the 70 lbs, but I wouldn't count on it personally.

Hope this helps.