I always day hiked Whitney, but I can answer a few of your questions. Backpackers routinely leave tents and heavy packs at Trail Camp and summit with day packs. Bears at TC are quite rare. I think there was one report many years ago. However, marmots will chew through tents and packs to get at any food stuffs. I think some people leave tents and packs open so the marmot can sniff around without chewing holes. The posts may be slow, but not the trail. You will have plenty of company during daylight hours and overnight at TC. We always left the Portal at midnight around Labor Day. Even then, there would be the occasional headlamp coming up behind us or stragglers coming down. Both Outpost and Trail Camps had plenty of tents. If you intend to break camp and hike out the same day you summit, then leave for the summit at first light.
