Best permalink structure for multisite

Hi! I know this question been asked a lot. But what is THE BEST permalink structure for multisite? Subcategory version of multisite.
Help me clear this out.

1. http://www.example.com vs example.com - is www version treated by search engines as a subdomain?
2. example.com/?p=123 - default permalink structure helps generate page faster, than other types of structures?
3. example.com/postname/ vs example.com/postname.html - is first example treated as 1 level deeper by search engines, than second example.
4. example.com/postname/ and example.com/postname.html - will they lead to duplicate content, when search engine index category? Because when you visit category you get example.com/category/postname/ and example.com/category/postname.html. As you can see those two are 1 level deeper. Same content with 2 different links, is it bad for SEO?
5. Should i permit the ability to change permalink structure for users? If they start to experiment with permalink structures, it will end up with a lot of 404 errors? And will it lead to ban by search engines?!

I'm just confused by all this info.