Adjust Wordpress theme for IE9, IE10 and mobile - repost

I have built a Wordpress theme - starting with the twentytwelve theme and modifying it considerably. It looks great in all desktop browsers except IE9 and IE10 - on those two the nav menu is too tall (height looks about twice what it should be) and the header looks too tall as well. Seems as though it should be very simple to fix those two issues. Aside from that, I'd like to adjust the theme such that on screens < 768px wide, the sidebars both render in the same column as the site's main content - so the screen would display center content first, then left sidebar, then right sidebar.