I can't answer for TGC, but I will give you as much info as I know at the moment.
"* I already offer Windows versions of some games, so exclusive across all platforms would not be possible. Can exclusivity work for non-windows apps? Just one device's version?"
As far as I know you will HAVE to compile each version for each device separately for distribution. you can only send your game to each device using AppGameKit itself (when the players are released. I wonder how much longer for those?)
"* I don't have a mac or any other of the devices - does exclusivity mean any form of testing there, or is it unrealistic for me to expect to program on an iPhone (for eg) without an iPhone?"
Probably hard to test your game otherwise, but I am not sure exactly what TGC will be offering for this. As a fair few will not have a mac to develop on they may offer the service to get it working on apple devices. Although game controls etc would be your job and without a device to test on would be difficult I think.
"* Will you be offering any sort of storefront (ideally, customizable) for specific products of ours? this way, we can send clients from one site to a (nearly identical looking) sales page."
I assume they will use whatever storefront they get within each appstore, but they may have plans for their own main front end, and link to each store possibly. If that's allowed.
Just my ideas on your questions, I'm sure that someone from TGC will be able to properly answer your questions at some point.
