From what I know, the admin interface is nearly (like default frontend, BTW) completely CSS styled, so you probably don't even need to mess with templating.
The configuration of toolbars in admin is somewhat complicated, I'd rather just use CSS in your case.
Getting rid of the right menu (actually moving it to the left) isn't hard at all. The hard part is extending the middle section to fill the gap.
The problem you run into is that the "maincontent" DIV has margin issues. To clear the left menu, it has a left margin of 19.5em (when the left menu is set to "medium"). This means that it won't sit alongside the left menu if you increase the width.
It gets even more complicated than that. I spent about an hour with it, and finally deleted all my overrides and went back to the old version in frustration.
If anyone has done this, I'd love to hear from you.
