Sorry for a late reply. Are you still looking for a way to do this or have you figured it out? On the chance you still need help…
I would first make a complete DB backup because my following plan is untested. Get into phpMyAdmin. You will see there are a number of tables of whose basename each site has in common, only the number after the prefix is different. Drop all tables that are lacking a number after the prefix whose basenames are common to all sites. Retain tables unique to the network root like wp_users, wp_blogs, etc.
Determine which tables belong to the “/us/” site and rename them, eliminating the prefix number, so that the tables take on the names of the tables you just dropped.
Install the Interconnect/IT DB search and replace tool. Use the tool to search renamed wp_posts, wp_postmeta, wp_options for URLs containing the “/us/” URL term and replace with a version without. I.E. search for “opheila.com/us/” and replace with “opheila.com/”
Remove the /us/ site record from wp_blogs. Note the blog ID. In wp_usermeta, find meta_keys starting with the old /us/ blog table prefix and replace them with the new table prefix. Find any of the old blog ID as meta_value, whose key is primary_blog, and change the value to the new blog ID (probably 1)
