Question on Multisite

Hi,

I want to clarify some questions about Multisite support.

Here is my situation:

1) I have quite a large database. So I want to create multisite with subdomain to reduce load for wp_posts table.
2) All the registration is handled by the main site. Subsite is used for content only. There won’t be any registration form on the subsite.

My objective:
A user’s experience on multisite is just like on 1 site. He/she registers, login, surf the content without noticing that there are multi-sites.

My questions:
1) Can a user register/logins in main site, and remains registered/logged in on the sub site?
2) When a user pays for level 1 account, can he/she has the same level across all the site?
3) Will the php condition such as current_user_is(“s2member_level1”) work across all the site?

Thank you.