For the stop to Silverthorne CO, Moab would be a possible stopover point. Although it's a small diversion off I-70 on 191, it's a lively little town next door to Arches NP that would also be worth a look. You could rejoin I-70 by going back up on the 128 which will take you a little longer but is scenic and will take you past Castle rock and Fisher towers.
Not only is Zion one of the easiest parks to visit from I-15, it's spectacular and it would be easy to spend a few hours there admiring the canyon, so you will have to consider that in your timing.
If you decided not to head through the heart of Death valley then you could head across from Big Pine on CA 168/ NV 266 to US 95 through Beatty to Vegas.