I think the whole 8 month thing, is just the dev's way of showing how the Happy Trails Caravan knew what they were doing, and once you were on your own, things didn't go as planned. I sure would like to know what happened along the way, though. There's quite a bit of stuff to do between Zion and the Mojave. :3
Well, that's just not true. For one, unless your character had an intelligence bellow 4 finding the Mojave would be easy due to road signs, basic sense of direction, etc. if they did get lost, I seriously doubt it would be a 7 month and 2 week delay for a 160 mile trip. Strictly between Vegas and Zion, there probably isn't much to do that would be of interest to a wastelander anyway (I don't think many wastelanders would really be into much canyoneering).
Also, it's a bug. Not everybody gets an 8 month long time interval.