I was there in September and didn't bother with reservations for the Aguille du Midi or the Helbronner cable cars, but just bought on the day of the trip. It may be busier in August, but I'd probably still just buy when I get there, perhaps the day prior to the trip.
Could you elaborate on what "high altitude treks" you plan on doing? I doubt that you'll be doing those from the Aguille du Midi as it's on a pinnacle of rock as I recall. You won't be able to go down to the glacier without a guide. There are lots of hiking opportunities around that area at lower altitudes so "high altitude" won't be required. The type of clothing to use will depend on where you plan on hiking.
One other very important thing to mention is that you'll have to give some thought to the issue of "altitude". The top level of the Aguille du Midi is at about 12,600 feet and that may be an issue for some people. Between Pointe Helbronner and the Aguille du Midi, I found that it was tolerable for a few hours but beyond that I started to have problems. The effects I experienced are not something I'll soon forget.
With the short time you have in Chamonix on the tour, the Aguille du Midi is probably all you'll have time for. You could also book a couple of additional days post tour, and do more exploring in the area or try Paragliding in the French Alps.