I don't think you'll have a choice about taking the shuttle if you want to manage your time. Yes, take the shuttle - it goes over to the Moro Rock area, then takes you on to Crescent Meadow. This is possibly the heart of the Giant Forest and definitely worth seeing.
Shuttles come about every 10-15 minutes, entire loop will depend on how long you take at each spot (I'd allow 3 hours and see Tharp's Log from Log/Crescent Meadow). Are you wanting to go to the top of Moro Rock? You can do that and see Crescent Meadow in maybe 2 hours if you don't spend much time at the meadow.
It's my understanding that the road to Moro Rock is closed until September to regular traffic.
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