This is basically a geography problem. First of all, Zacatecas is one of the most difficult spots to travel by bus from anywhere, because it is among several mountains. Saying that, they don’t have good airports, neither well connected to any of the major cities. Still, the trip you are describing would take you driving (which is always faster than taking the bus), good 24 to 30 hours driving non stop.It is not possible to find what you are looking for, and even you could connect several dots to do it, your in laws will spend at least 2 days in the road, probably even more (it is really far!!!)

Second. Most of bus companies in Mexico are regional, they are only a few ones that may cross the country, and they do it making stops, there’s no other way. There are better bus companies in Mexico than in the US (like ETN, Primera Plus, Peninsula, etc), but none of them will make a trip like the one you are describing.

Basically this is the logic in Mexico, you take a bus for most of the trips no longer than 4 to 6 hours. Then you get great comfortable non stop trips. Beyond that you can get competitive airlines for those other trips. People who cannot afford to travel by airplane beyond 6 or 8 hours at the most, means they need a cheap option to travel, then you have second class multiple stop buses to cover those needs.

I recommend you to look for airplanes flying to Guadalajara, and from there they could take a non stop 5 hour bus (Omnibus de Mexico or Primera Plus) to Zacatecas. There are several non stop flights from LA to Guadalajara everyday (up to 4 or 5). If you want to make it cheaper, they can go by bus from LA to the Tijuana airport, and then you could get several more flights to Guadalajara.