I will see if I can reproduce what you have done.. It might take me a little while..
There is surprisingly little on the internet.
I actually thought the Mac would work ok since it is using SAMBA rather than true SMB.
What OS are you running on the Mac??
What happens when you try with windows computer?
Clearly windows access is more difficult than AFP.. and unfortunately I really don't know where you can get AFP protocol for windows.
Remote access to airport disk from Windows
Airport Extreme attached HDD remote access (mac to Mac)
Can you please open the other ports.. 135-139 but don't translate it.. and see if that helps.
In fact just briefly and I mean like absolutely min time.. try opening the ports as is.. no translation and see if it works.. if not then translation has nothing to do with it.
I am going to run out of edit soon..
Are you using the LAN of AE address to map to??