Move main site into subfolder (htaccess) - php

Question

Need to organize several sites in one hosting account, putting each into its own folder.
Via CPANEL, this is completed and working for the add-on domains, but the main site (with all its subfolders) also needs to be in its own folder.
I have been doing this via PHP:
example.com/index.php :
<?php
header('Location: subdir/mainsite/index.php');
?>
The down side is that there is a page flash as the site redirects.
Another downside is that the URL reflects the new location in the address bar:
http://example.com/subdir/mainsite/
I just want visitors to see http://example.com
I suspect this can be resolved by using htaccess instead of PHP.
So, if my domain is:
http://example.com
And I place the main site files in:
public_html/_sub/mainsite
And the add-on sites must continue to work...
What would the htaccess look like?
This is what I have thus far:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteCond %{REQUEST_URI} !^/_sub/mainsite/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /_sub/mainsite/$1
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteRule ^(/)?$ _sub/mainsite/index.php [L]