A few things:

First, your odds will be much better if you can go Sat-Sat instead of Fri-Fri, if for no other reason than season service to ACA from MSP will be running on Saturdays during March. CO and DL may also have Saturday seasonal service, although I don't know.

Second, also look at ZIH, ZLO, and PVR, which are along the Pacific coast, north and west of ACA. 127, 356, and 450 miles respectively. (Driving 450 miles in Mexico would likely be an adventure, but if you really want to do it...)

Third, although they used to, I don't believe that AeroMexico flights currently show on nwa.com. I don't think you said whether or not you searched online or called, but there's potential there if you just used the website. There are daily flights from MEX and GDL to ACA on AM. There's definitely regular availability on the 3d from DTW to MEX. And regular availability out of GDL on DL on the 10th to Atlanta. If you haven't called yet, go ahead and try...if you get a good agent and they don't show any availability on NW/CO/DL, ask them if they can route you through MEX or GDL onto an AM flight. And AM also flies between a few US cities and MEX also.

Basically, if you're open to the possibility of 2 or 3 connections and flying someone other than NW/CO, you have a decent shot of finding something.

Finally, it depends how much of a gambler you are, but it's not necessarily accurate to say that if there's no regular availability now, that there won't be any in a month or two or three.