The track due-South out of Mesquite leaves town crossing the river where there is no crossing. You can cross the river a mile East on Scenic Blvd. Earlier that same track leaves the freeway frontage road on a sketchy road along the side of a golf course. 1.3mi East of the golf course is the main road (Scenic Blvd) that crosses the river, which would be a much better way to go.
The track South of Bar-10 goes off the edge of the Grand Canyon cliff on a hiking trail. No road or rideable section that last mile or so!
Your track due-South out of St George is blocked by a new freeway that goes to the new airport there. Where your track first hits dirt, just stay on the pavement 2 miles more, crossing the freeway at an underpass, then you'll hook back into your track there.
Your track going East out of St George goes straight through someone's house and their back yard fence where there is no road, 3 miles before reaching Sand Hollow Reservoir.
Leaving Sand Hollow Reservoir South, your track goes where there appears to be no road or trail, bushwacking through the bush, about 3 miles after leaving the Sand Hollow area.
A section of your Lime Kiln-Mt Trumbull track goes through a cattle fence where there appears to be no way through. It looks like there was a road there at one time, but not anymore. The blocked part is just West of a dirt road called County Hwy 257 exactly 3 miles South of Mt Trumbull school.
The exact point where your -Toroweap track exits Hwy 389 in Colorado City is no exit, and probably goes through a fence. Exit 170 yards North of that point, and hook right into your track.
Your Pearce Canyon track goes through a nasty rocky wash where there is no existing road, going South from this point ( 36° 16.617'N 113° 54.420'W ). It won't be any fun unless you enjoy torture. Also, there is a BLM fence right at those coordinates. People have ripped the fence open there, but it could be back up when you arrive. There's a much easier way around, on the road 2 miles West. When you arrive at the abandoned cabin, go another 50 yards down the road and turn LEFT up a beautiful smooth wash. That will hook you back up with your track.
Does your topo map say that this point ( 37° 2.931'N 113° 45.196'W ) would be fun to see, just 1/4 mile East of your Lime Kiln-Mt Trumbull track on a powerline access road?
Hope you carry a LOT of gas, because most of those dead-ends are a LONG ways from any gas station. Also, unless your're amazingly fast, you won't get in and out of some of those points in one day. Make reservations at Bar-10 and also camp at Toroweap and one of those points West of Bar-10 and you might get away with it. Good luck!