This can get tricky with the copy protection on DVDs, but it is possible to do!
1. I use handbrake to rip DVDs, but you do have to find extra software to allow copy protected DVDs to be ripped on Windows. I think on OS X it's still built in.
2. To stream video to the iPad, I like Airvideo. You set up a little server (no difficult configuration) on the machine you keep the video on, and it will stream and convert the video to the proper bitrate and size for your device. There are other apps, but I've found AirVideo to be my favorite. As far as Netflix goes, there's a dedicated app for the iPad. I believe there's other ways to stream it, but they're usually difficult to set up and don't work as well. Youtube also has its own app on the iPad. As far as using OS X goes, you should be able to alias the video folder into your movies folder and it should be accessible through front row.
I'm not sure about the Apple TV question as I use Windows Media Center for my needs.
3 and 4, I avoid iTunes for anything more than syncing my iPad/iPhone and managing music personally, so I can't give a good answer to these questions. I believe I've read that it's possible. Unfortunately I don't remember much more. Wired and Lifehacker tend to have good articles on the sort of stuff you're asking for though.