I would suggest being an ickle bit selfish and work out a plan to suite what you would be happy doing and then see who is willing to sign up. You can tweak it a bit afterwards if needed but if you try to please everyone you will be going round in circles forever!
I can definately recommend September - we went for two weeks starting the 17/09. The weather was good (a couple of cloudy days) and we did two waterparks plus Discovery Cove quite happily and it wasn't too hot which might make the theme parks really hard work. US, IOA, BG and SW were all really quiet, we found Disney to be more busy - particularly AK but still ok.
As you want to do lots of non disney things and intend on having cars I'm thinking staying off site may be a better (and cheaper) option for you, or at least a split stay. We stayed on I Drive and really enjoyed it - close to Universal and Seaworld (and Aquatica when it opens) and we enjoyed the more adult atmosphere at CityWalk and I Drive for nightlife. We do prefer the other parks to Disney though so this obviously this effects things!
If there are 8 of you maybe a villa is the way forward - or are you dead set on staying on-site?