This may sound dumb, but you might want to try setting the AHP software to transieve ALL housecodes. After reading all of this several times, it really does seem that there is a connection problem between that specific switch module and the CM15a. As you state, the AHP software IS activating the screen module. This means the CM15a IS getting the RF signal. It probably is sending the PLC command out, but not correctly. Your workaround should work just fine, so maybe you should leave well enough alone. I find this a very strange problem, that I have seen before with my setup, but never totally got rid of. It also seems to come and go with battery level in my palmpad, even though the RF signal still shows up in AHP. (Weird, huh.) How the AHP software can correctly send the PLC command when asked to from the Mouse/screen but not resend the command via PLC when getting a good RF signal and having the unit set to Send/receive the correct housecode makes no sense to me at all.
IMHO, it can't be Noise, or ANY external interference. Mouse OK & RF Receive in activity Monitor proves this. This must be in the CM15a. If the Palmpad had the problem, the TM751 would bomb too. Good Luck and if you figure this out, please let us know. I'm sure more than just me will be VERY interested.