It breaks my heart but I'm going to say something rather bluntly, and it is speculative, but.... I bet none of the dead were wearing full-faced helmets and/or any other practical riding gear.
In this case, with a vehicle traveling at them at a decent rate of speed, I don't think it would have made much difference for the lead riders. A friend died by riding into the back end of an SUV at 70, wearing full gear.
In regards to what the best case reaction scenario would have been, I choose C and B, for both the oncoming driver and motorcyclists. Slow down as rapidly as possible in a controlled manner, pulling off the road as far as safely possible. In the desert, do your braking on the pavement, before pulling onto what could be a sandy, slippery shoulder. Signal as you slow.
In regards to the "Stop or Chase" question, always stop. Especially if you are a witness. Even if you don't have medical training, you can always shield someone from the sun or wind, get water, comfort uninjured riders and observe them for shock, put out fires, stop/direct traffic, get descriptions of the fleeing motorist, etc. Regardless of what they ride, they're fellow riders. There's always something you can do for them.
In regards to what to do if I was the perp, Pull off the road, stay in my car. Call CHP. Stay on the phone. If I felt I was in danger from the remaining riders, I'd tell the CHP on the line of my fear & drive up the road until I found an officer to flag down.
I'm sad for the riders & their families, but I'm more sad for the speculators who think the riders could have been responsible in some way.