You can modify the system launchd.plist that's bound to this so it launches your custom apache install.
You do this by editing the:
/System/Library/LaunchDaemons/org.apache.httpd.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Disabled</key>
<true/>
<key>Label</key>
<string>org.apache.httpd</string>
<key>ProgramArguments</key>
<array>
<string>/usr/sbin/httpd</string>
<string>-D</string>
<string>FOREGROUND</string>
</array>
<key>OnDemand</key>
<false/>
<key>SHAuthorizationRight</key>
<string>system.preferences</string>
</dict>
</plist>
Change the /usr/sbin/httpd string to the path to your custom apache install. Make sure you disable Web Sharing first or from the command line:
launchctl unload -w /System/Library/LaunchDaemons/org.apache.httpd.plist
After you edit it, either click the web sharing button or from the command line:
launchctl load -w /System/Library/LaunchDaemons/org.apache.httpd.plist
For the second question, you can set a redirect in your apache config
/path/to/apache2/conf/httpd.conf
Not exactly sure where this is for MAMP, the general syntax is:
Redirect / http://mydomain.com:591/FMI/IWP/
It's usually good practice to wrap these in conditionals
<IfModule alias_module>
Redirect / http://mydomain.com:591/FMI/IWP/
</IfModule>
And I think it'd be a more elegant solution to proxy pass the requests
</IfModule>
<IfModule proxy_module>
ProxyRequests Off
<Proxy *>
Order deny,allow
Deny from all
Allow from localhost
</Proxy>
<Location /filemaker/>
ProxyPass /filemaker/ http://www.google.com/
ProxyPassReverse /filemaker/ http://www.google.com/
ProxyPass /images http://www.google.com/images
ProxyPass /extern_js http://www.google.com/extern_js
ProxyPass /intl http://www.google.com/intl
ProxyPass /csi http://www.google.com/csi
</Location>
</IfModule>
In this example, I'd just have to go to http://localhost/filemaker and it shows the google page. What resources you pass in the ProxyPass depends on what filemaker needs.
If you don't care about retaining your domain and you want everything to proxy, you'd
</IfModule>
<IfModule proxy_module>
ProxyRequests Off
<Proxy *>
Order deny,allow
Deny from all
Allow from localhost
</Proxy>
ProxyPass / http://www.google.com/
ProxyPassReverse / http://www.google.com/
</IfModule>