Hi Dee - you'll be launching a day after us.
The Clay Hills road is okay for a street-clearance vehicle, if you take it easy. AND...if it's dry. We got into a situation last year after severe downpours washed the road out in places - there was a 50cfs river running through one of the washes, and several spots were pure quicksand. We and another group spent hours shoring up the road in two or three places. Before you go down there, check around with the sheriff and locals about the condition of the road - if any doubt, don't take a car down there. It'd be a long wait for help if you get stuck, and expensive (although our cell phones did get a little spotty service on the CH road.)
I wouldn't do the Dugway (SH 261) with a long trailer. No way.
We're trying to figure a way to do shuttle ourselves, but we only have two vehicles and two rafts, and I'm a little concerned we may not be able to get both rigs back in my Ram.
Are you launching from SI on the 10th, or from MH? We may be able to help each other out. Drop me a note.