Re: one theme, one widget setup
To get the theme that you want all users to have, just make the theme you want the default WPMU theme. This can be done by naming the folder of your theme “default” or looking at a plugin like this plugin.
To get all the widgets that you want in there, It would be best to actually hardcode the widgets you want into the theme itself. If you look at the code of certain themes that do this already, it shows you how to do that. Then, finally if you don’t want users to be able to change around widgets, there is a plugin that limits what menu’s users can see, but don’t have the link off hand. You will have to search for that one.
That is what I would do anyways!