wow, you actually got it to receive data from RR in your script (I'm impressed) .. interesting that you were unable to send ..
Anyway, keep in mind that I don't much at all about autoit scripts. Still, looking at your code, you're doing +1 to the string length which is probably adding an extra "garbage" character to your command being sent.. this could prevent RR from interpreting the command from what it's supposed to be. A quick test would be to send a multi-command like PLAY||NEXT chances are that if the garbage is the problem, the first command would work and the second would not.
In any case, I hope you realize that the SendCommand function you made would have the same effect as using exec.exe (perhaps just better for commands with: >> || and ; ) -- something you can overcome with exectbl.ini.
I also hope that you're aware of the SENDMSG command in RR that would allow you to send data to your autoit script directly from RR's skin (pressing buttons etc) -- this is just a tip
PS: If you get the SEND script to work, I'd like to include those scripts to RR's SDK examples with your permission.