For the extra permit, start checking Sunday at 8:30. People dropping off a slot or two may occur later in the day, though, as people like you come in and pick up their permit.
Never heard of the camp sites at Horseshoe Meadows being full -- they are all one-night-only, so people don't set up and stay. I've even arrived late at night, and just walked out to a flat spot and set up.
Trail Peak is an easy warmup hike from Horseshoe Meadows. Head south to Trail Pass, then west to the peak.
Trailhead walk-in spots are right by the trail head, so that's the best advantage. Pretty much always sites. I've never camped in the ravine area.
