Trying to install the wonderful Concrete5 CMS to use as my main site engine - the problem is it has about 15 different files and directories, and they clutter up my root. I'd really like to move it to a /_concrete/ subdirectory, and still maintain it in the domain root.

htaccess has never been my strong suit - after a lot of research and learning, and a lot of error 500s, my frustration is overriding my pride and I'm posting here. Here's exactly what I'm trying to accomplish:

• Any requests that come through www.domain.com are forwarded to www.domain.com/_concrete/, except in the case of an existing file.
• The end-user URL shouldn't change - they will still see the site as www.domain.com, even though they're being served www.domain.com/_concrete/.
• Multiple subdomains exist on this site as sub-folders within the root - thus, only requests coming through www.domain.com should be redirected.

Here's the closest I got with my htaccess, which produces an error 500:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.com [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !^_concrete
RewriteRule ^(.*)$ _concrete/$1 [L,QSA]

This is the result of 4 hours of sweat and blood (mostly blood), so I have to be close. I'm hoping one of your fine minds can point out a stupid mistake and put this thing to rest swiftly. Thanks for your time!

Addendum: I previously posted .htaccess - alias domain root to subfolder a while ago, which got me started. Please don't fall into the trap of thinking it's a duplicate.