htaccess rewrite rule if file exists else go to PHP file - apache

Question

I'm trying to write a rewrite rule in my .htaccess that redirects all traffic except one domain to https. Then if the file it is trying to go to does not exist, then run the last block that follows, going to loader.php, but if the file goes exist go to it.
Here's one of the iterations I have so far:
RewriteEngine on
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} !^(www\.)?(korle\.tt) [NC]
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [NE,L,R=301]
<If "-f %{DOCUMENT_ROOT} . '/'">
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ /loader.php [L]
</If>
<Else>
RewriteEngine On
RewriteRule ^ /404.html [R=404,L]
</Else>
What I am trying to do is if the user goes to korlett.org or korlett.net or korelett.com or any file within, it redirects to https://. But not http://korle.tt or other domains pointing to this dir (for now not specified), they'll stay HTTP.
Then after that, if the file does not exist or if the URL returns a 404 then run this:
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ /loader.php [L]

Answer 1

then if not sent it to loader.php to check if there is a long url pairs with that fragment/"dir" and if that fails then serve a 404 page.
This 404 response would need to be managed by loader.php - ie. in your PHP script. Because once the request has been passed to loader.php (any request that does not map to a file or directory) then .htaccess has effectively finished with the request and you can no longer get Apache to handle the 404 response.
It should serve a 404 if loader.php isn't found
However, this can indeed be managed in .htaccess, by setting an ErrorDocument and removing the slash prefix on the RewriteRule substitution (which was probably the intention anyway, since you have included a RewriteBase directive). Removing the slash prefix prevents a rewrite loop (from repeatedly rewriting to loader.php). In other words, instead of this:
RewriteRule ^.*$ /loader.php [L]
It should be this:
RewriteRule ^ loader.php [L]
The regex ^ has the same net result as ^.*$ in this example, and simply serves to match anything (like you have in your HTTP to HTTPS redirect).
This prevents a rewrite loop because when rewriting to loader.php (relative path, no slash prefix) the rewrite engine detects that you are rewriting to the same URL and prevents further rewrites. Allowing the Apache 404 ErrorDocument to be served. (Whereas /loader.php is technically a different URL - as far as mod_rewrite is concerned - so the rewriting would continue.)
So, your complete .htaccess file becomes:
ErrorDocument 404 /404.html
RewriteEngine on
RewriteBase /
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} !^(www\.)?(korle\.tt) [NC]
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [NE,L,R=301]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ loader.php [L]
Another way to prevent a rewrite loop is to include a specific exception for loader.php. In fact, this can be used together with the above, in order to make the rewrite fail early, rather than waiting for the rewrite engine to detect the no-change. For example, change the last block above to read:
RewriteRule ^loader\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ loader.php [L]