How do you have MAMP Pro setup? I clicked the default ports and I also switched from user/user to www/mysql as recommended by MAMP. Though I am not sure why, it said for security reasons.
So my ports are 80 and 3306. I didn't do anything in Dynamic DNS or Postfix yet.
I see that you can type in http://name/ for local testing and then add the .com for remote view. But I still have to set my url prefix in site settings to http://localhost/name or I don't get a preview in the browser for the local.
One note, I am doing all of this on one site only now. It's my php sandbox folder and it's now located in the MAMP/htdocs/ folder. Maybe that's the difference. Should I keep it in the Sites folder instead?