You are hitting on two things:
1) Apache conforms to the HTTP/1.1 spec for content negotiation (http://httpd.apache.org/docs/1.3/con...gotiation.html) which means that when the browser specifies a preference for English documents, Apache will prefer index.html.en to index.html

2) The DirectoryIndex directive gives a prioritized list of files to be used when a directory URL is received: http://httpd.apache.org/docs/1.3/mod/mod_dir.html

As I said earlier, you really do need to read the docs if you are doing anything out of the ordinary.

I think that is its rather-too-subtle way of saying that you need to run it with root privileges - try prefacing the command with 'sudo'

You can, what you can't do is run the same file extension through both versions.

WHY you'd want both of them at once is another issue.
Orthodontist: how did you get PHP5, and are you sure PHP is working?

Create a file in the /Library/Webserver directory called "phpinfo.php" containing the following:

and load it with :
http://localhost/phpinfo.php