Hi,
Just my opinion here:
1. It depends on your public. If you're going to do a site accessible by a lot of people then you have no choice. All of them, including Safari which you forgot. But, if you're working for a customer that only uses one browser and you don't have the time to fix all the compatibility issues then... You decide.
2. I think it will render, but you won't be able to really use HTML5 functionalities. I'm not really sure though.
3. Where do you get that? I'm not sure this is true, take a look at this:
http://www.ie6countdown.com/[
^]
Hope it helps