Your acclimation plan sounds good, and based on the peaks you've hiked recently I wouldn't worry about your fitness level, especially if you are splitting the hike up over 2 days. I really like Onion Valley, my only recommendation would be to not overdo it on your warm up hikes. One year I leisurely hiked the Kearsarge Trail to Heart Lake (at about 11,000 feet elevation) and that was perfect, another year I hiked a bit too energetically to Kearsarge Pass and my legs were spent after that and were not fresh on my Mt Whitney hike 2 days later. Make sure to drink a lot of water in the days before your hike...the last times I camped at Onion Valley I drank ~4 liters a day. On your Whitney hike I would recommend electrolyte tablets, and Nuun tablets in particular.
The stairclimbing you've been doing will be really helpful I think. My least favorite part of the hike is after Mirror Lake where there are lots of stairs/steps...my last minute training (I have a day hike permit for 8/31) has been to focus on stairs specifically to prepare for that part of the trail. I've been going to Baldwin Hills Scenic Overlook (Culver City Stairs) a couple times a week, and I'm hoping that that will help.
I haven't camped at Trail Camp before, only Outpost Camp, and I left my tent up. No marmot issues there. A friend camped at Trail Camp once, and left his tent open as some recommend, and marmots peed and pooped on his sleeping bag. If I were ever to camp at Trail Camp, I would probably just keep the tent staked, but take the poles down and put some rocks on on top of the tent.
Have fun!
