The themes aren't changed in any way, other than to ensure compatibility with MU, widgets, etc. The basic stuff that 99% of MU admins will do to a theme to tidy it up for multiple users.
If the theme is internationalized to begin with, it still is.
Without being logged in, you can do anything in terms of viewing a plugin with the exception of downloading it.
The video tutorials are pretty decent for novice users, and like I mentioned originally, the plugins are great if you want/need them.
Andrew has been great with support, and fixing any issues with plugins (that are the plugins fault, of course), as well as being open to ideas and suggestions for new plugins and features.
If there is value in that to you, it's well worth the subscription.