Well another option comes up if you want to skip the two/three days on Whitney. Continue on the PCT from Crabtree to Rock Creek crossing nice camping (albeit tromped down) and bear boxes there just after crossing the creek. Depending upon your time/energy you can continue up hill toward Soldier Lake - bear boxes there as well - (heading for Army Pass) near timberline. You can spend a day in and around the Miter area. Also spectacular.
Then you have a choice of going south around the much lower Cottonwood Pass (to Horseshoe Meadows is two days from Rock Creek with last night overlooking Big Whitney Meadows to the South) or up over Army Pass and an option for going up Mt Langley for your 14'r. Then down via either OLD or NEW Army Pass. If you go down OLD army (I can send you pictures from the top looking at now abandoned trail - its a help) you save a 500' (or so) uphill to NEW Army Pass and you get to visit most of the Cottonwood Lakes below you. Spectacular view from Old Army Pass. There is camping between Lake 4/5 just as you exit the pass area.
If you go over NEW Army Pass you get to see the edge of first two Cottonwood Lakes plus Long Lake on the way out. Camping at Muir Lake just north of Lake 1 is nice if you want to extend your trip. There is a 'use' trace trail between Lake 1/2 to get to Muir.
Both trails over the passes end up in Horseshoe Meadows.
The trip from Onion Valley to Horseshoe Meadows is about 5 days total - not counting Whitney. And you get to see some of the finest high altitude Sierra.
You end up about the same place as Whitney Portal (about) just south of it. You can probably get a quick ride down the hill or you can pre-plant your car there.
OR!
Stay near Tyndall Creek as your base camp and wander down the Kern River visiting some of the most unvisited high country -- the west side of the drainage. I am going to guess that not more than a hand full of people see it. Most are on the way to or from Whitney and don't want to 'waste' their time there. You can go all the way to (Kern) Junction way down at the bottom. Also get into Wallace drainage and other lakes and high cross country walking on EAST side of the drainage.
OR!
You could day hike Whitney from the west side (longish day 4AM to 7/8PM) retrace your steps to Tyndall Creek, and then skip out over Shepherd perhaps the next or following day down to a hitch back up to your car at Onion. There is good camping near top of Shepherd on the west side - no bear box. It would be a fairly easy day from Crabtree to Tyndal and back out over S. Pass the following day.
Would probably add an extra day to your trip. Up Whitney, back to Tyndal (could stash some food -- cannister -- and weight from prior crossing) then added day over pass down. A beautiful place to have to retrace your steps - Bighorn Flats.
If you do that, then one of you might want to leave your car at Shepherd Pass trail head after dropping your heavy packs off at Onion. Then catch a hitch up to start your hike from Onion. Lots of early morning traffic going up hill to Onion.
OR..
