URL Rewriting is Sending Requests for Non-Existent Pages to Main Page, Instead of 404 Page - apache

Question

I'm trying to set up some URL rewriting using my .htaccess file. I'm a beginner, so bear with me! What I'm trying to do is reroute all requests from the root to the /main/ folder, since that's where I've got my CMS's files. For example, www.example.com/page should request www.example.com/main/page, but just show up in the browser as www.example.com/page. There are times, though, when I want to be able to access other folders, like when I post a dev or stage site at www.example.com/dev or www.example.com/stage. In these cases, we just can't have the request rerouted. Here's what I have so far:
RewriteRule ^(/)?$ /main/ [L]
to rewrite root requests to /main, and:
RewriteCond %{REQUEST_URI} !-f
RewriteCond %{REQUEST_URI} !-d
RewriteRule ^(.*)$ /main/$1 [L,QSA]
for all the other requests. It works fine, most of the time, but the problem is that if I type in www.example.com/something_that_doesnt_exist, it shows the main page (i.e. /main/), and keeps the URL in the address bar. If I take that same URI and add /main/, so in this case www.example.com/main/something_that_doesnt_exist it goes to the 404 page, which is the desirable result. How can I fix this so that www.example.com/something_that_doesnt_exist goes to the 404 page?
And, for bonus points, how to I get www.example.com/main to actually forward to www.example.com, so that if someone types it in it either goes to www.example.com (changing the URL in the address bar), or does a 404?