1. Yes, you can use the Gift Card to make purchases but everything you purchase from the App Store only works in OS X.
2. The Mac Store (Apple Store) is just a web site on the internet. www.apple.com/store. If you mean the APP Store, then you need to be running OS X
3. Mac Software does not run on Windows
4. Yes, but you still need a Mac. Without the Mac, you cannot access the App store.