Hi there, Wild Bill!
Setting 4 different ATCs on the 4 channels IS possible but only through the mission editor, NOT while in the cockpit. OK, I guess you knew that already. But then how about using the radio succesfully? Well I can tell you from my own experience that if the mission file you're working on has been used by a totally different aircraft previously, then the radio comms will most probably be fubar.
What to do? You got 2 options.
1. Create a brand new mission file or...
2. Open the .miz file with winrar and look for a folder named "radiocomms" or something (can't remember exactly) and just delete the damn thing. Had to do that for all my separate versions of the Outremer-campaign. Then I got the comms working.
Homing radio?
Never heard of it, sorry.
Shady compass?
I can't even tell, I only use it for reference. Map reading is safer.
EDIT: I'm home now! Here's a shot of the folder I mentioned. I tossed it and then I could redo the mission for other aircraft's radios. Strangely enough, I couldn't find this folder in following missions created from this "mother"-mission.
No clue as to why...