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. vs – is www version treated by search engines as a subdomain?

2. – default permalink structure helps generate page faster, than other types of structures?

3. vs – is first example treated as 1 level deeper by search engines, than second example.

4. and – will they lead to duplicate content, when search engine index category? Because when you visit category you get and 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.