tr909,
as you wrote, the diode is to protect the optocoupler from reverse voltage. i didn't test it without, but it should work.
other optocouplers should works as well, as long as they aren't much slower. there should be graphs or numbers in the dateasheet. i used a 4n28 because i once bought a couple of them for another project.
it might be, that my drawing of the midi socket is misleading, or rather that it shows the plug, not the socket. i'm sorry for that. it was a quick drawing when it worked for me. i noticed it some days ago when building another device, but forget to get deeper into it. simply try reversing TX and ground on the socket. please remember the diode in that case because of GOTO LINE 1.
i started on a midi library + real instructions with real images, but this will take me a while. please let me know if you could get it working.
kuk