Make the “Add New Post” one column for all users

This is a tricky one, there is a filter called meta-box-order_{$page} where page is the name of the post type so for post you can use:

add_filter('meta-box-order_post','one_column_for_all');

 function one_column_for_all($result, $option, $user){
    $result['normal'] = 'postexcerpt,formatdiv,submitdiv,trackbacksdiv,tagsdiv-post_tag,categorydiv,postimagediv,postcustom,commentstatusdiv,slugdiv,authordiv';
    $result['side'] = '';
    $result['advanced'] = '';
    result $result;
 }

this is untested!!! but should work.