Ah... it turns out I get to cement my rep as village idiot to Evilbore: I think I got the mechanic as Mattias, then restarted as Mui, and hadn't yet completed the "get mechanic" mission as Mui... So -- not a glitch.
However, Sho Nuff and I played a longish bout tonight and managed to get him stuck in the terrain, planted more firmly than a tree, in the race mission -- the one where you get the mechanic. Between starting us in different spots, limiting the distance we can separate from each other, a time limit, spiking Sho Nuff into "immovable object" territory, and the car's spongy Havok fizziks joi, we failed a mission five times that I passed the first try as Mattias.
So, yeah, the game's not perfect but I'm still having more fun than the reviews credit the game for, and more overall fun than GTA IV.