>1) why does it give a forbidden error when I try to view an html page that resides in cgi-bin?
Because files in cgi-bin are expected to be executed, not viewed. Especially if you've set the execute bit - you're telling apache to execute a .html file which isn't going to work.
In addition to that, you need to check the permissions on every directory in the path, including the /Installer directory. This needs to be at least 555 so that Apache can see the files in it.
>2) how do I fix this error (for a cgi file that's not in cgi-bin)?
Executing cgi's outside of the cgi-bin directory/path is not recommended. It opens potential security issues, especially if you allow other users to create web content.
However, if you really want to, you'll need to add an Options +ExecCGI statement to your httpd.conf, like:
<pre class=command><Directory /Library/WebServer/Documents/InsecureCGI>
Options +ExecCGI
</Directory></pre>
Now any URL of the form
http://your.server.net/InsecureCGI will be executed as a CGI.