UA is far more likely to "hold" the 2nd part of a direct flight for the 1st part, than a standard connection.

That in no way makes this schedule a good one, and I hope it gets fixed ASAP, but your above statement is very incorrect.

I am wondering...US DST gets adjusted on Mar 11 (along with Baja Mexico and parts of Chihuahua/Tamaulipas), while MEX gets adjusted on Apr 1.

Perhaps someone hasn't adjusted the scheudle to reflect the DST adjustment.

Let's do the math. Leaves MEX at 6am, flight time 4:50, makes arrival at 10:50. There is a 2 hour time zone change, so really, arrival is at 8:50, except that the US has already sprung forward, making it 9:50.

10:1 said whoever is doing the schedule didn't account for that the US and parts of MEX have differing observations of DST and needs to make the plane leave MEX an hour earlier to make it work.