Seems like you did a good deal of exploring to me ... ! I haven't ridden much down there (by comparison). Spent a lot of time, but the majority was with family ... working on a farm outside Mulege for a while, migrant labourer that I be... The head of the singletrack I mentioned is visible (if you know where to look) from the parking area at Mike's. You head down the "driveway" like you're going to leave Mike's & hang a left before you get to the river - the trail begins about 50 yds from there, winds around for a mile or so, then reconnects you back at the main road about 1/4 from the river down the hill from Mike's - makes a nice "cap" to the ride after coming up the fireroad.