No matter how you look at it, people who commit crimes don't want to hang around and get caught. They want to get away quickly--ergo a vehicle. I'd be really surprised if it was a hiker who committed this arson.
I wouldn't say Colockum is a gravel road most of the way. The last state highway map I looked at had something about it being a primitive road and not advised for passenger vehicles. It has some rough sections. You can do it in a 2wd but I never did.
I have been over and back on the Colockum Pass from Ellensburg to Wenatchee several times but not recently. I think that is part of the Stagecoach Road.
Ha, no probably not.
GPS isn't really necessary. Just gives you an idea of where you are in relation to any sign of civilization.
I've done it from that side too.