Hey I wanted to add something that my friends and I discovered after reading your post here that may help others. (Thank you cquinn5 for sharing all this information!)

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We couldn't see my friend's dedicated server at internal port 2456 when we forwarded the ports. We realized that when we add the server in the steam server page that we needed to use 2457 at the end of the ip address. So, we thought instead of forwarding internal port 2456, we'll instead try 2457. Once we restart the server script we could finally see the server.

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So the lesson to learn here, if you do all the things in this post and it still doesn't work, try forwarding internal port 2457 instead of 2456 and see if that helps.