1) Shopify has a very extensive API that is interacted with through json. Given that, the language we use for Shopify will have no impact on that language you choose for your Apps. Additionally, our Python library has had a lot of attention lately and has been brought pretty much up to par with our Ruby library
2) The distinction between Python 2 and 3 is no small difference. Python 3 is incompatible with Python 2 and thus the libraries that are written for Python 2. It's worth noting that Python 2 is going to be maintained for quite some time - most of the Python community is still on this version
3) As Python 3 matures our library will probably follow but as it stands it makes more sense for us to stick with Python 2 for the above reasons
4) I'm not extremely familiar with Django but from what I gather 1.6.2 supports Python 2.6.5 - so you should be good to go
5) Unfortunately I don't know of any recent and large scale applications that have been open sourced in Python/Django