To answer question 3, AndyMark is an obvious source.
http://www.andymark.com/product-p/am-0844.htm
I haven't shopped around to see if there's a better deal, but $89 for two doesn't seem too bad.
My opinion on question 4: The GDC wants to allow enough energy that we can do interesting and educational things with our robot, but not so much that it becomes unsafe. Allowing only one battery (or one of two nearly identical batteries) makes it much easier to accomplish that goal.