Do I understand correctly that you're going to start @ Onion, go over Kearsarge/Forester, then divert from the JMT in order to summit Williamson? And then you're going to backtrack/return to the trail in order to hit Whitney on the way out?
Have you considered just going up Shepherd, summit Willy, traverse the ridge-line down to Wallace, then go up over Russell-Carillon, down to Iceberg and then up the MR and out? If it sounds ambitious, it is. However, anyone who can solo Williamson should be able to deal with the cross-country challenges as well.
I've been up/down both Shepherd, Kearsarge & Forester a number of times. While I haven't done Williamson, I've been off-trail throughout the region.
Having done the Onion to Whitney hike myself, I think you'll find yourself deciding to skip Williamson. It's both out-of-the way and adds an unnecessary fatigue element to the equation.
If you don't want to traverse x-country from Williamson, then a much more straightforward & logical hike would to still go in @ Shepherd. Williamson (or Tyndall) are right there. Thereafter, the Shepherd trails joins the JMT @ Tyndall creek, making it a more logical continuation on the way to Whitney.
PS I've driven my Sienna mini-van 3 times to the Shepherd hikers' TH without any problems. Just watch out for rocks.
Last edited by Hobbes; 01/18/17 04:49 PM.