Hm. I'm not entirely certain what scenario you're describing here. Hm.
The first case : yes indeed. sy is just a variable, which can be either "true" or "false" in this case. I believe that there should be a "sensor" (hm. We call them triggers) which is large enough to encompass the entire enemy force, which is activated by "West not present" (=no west units in the area defined by the a/b values), condition "true" (=no other condition needed but the one we already mentioned) and putting
sy=true in the On Activation field. Basically, when the trigger is triggered, by you having killed or chased away all West units, the variable "sy" is true.
As noted, you also need the variables westcardead and uscaralive to be true. (I think it explains how to do that in the tutorial). When all of these are true, THEN the "endmission" trigger can be activated. There's nothing special in the variable name "sy", although I really have no idea why they chose to name the variable that
As to the other thing : uhm, once again, not entirely certain what the context is but : a radio trigger (Radio Alpha, for instance) can be set to either "Once" or "Repeatedly". You want to make it "Once", that way it'll disappear when you've called it once. Hm.
I wish I could show you some more tutorials *looks sadly at the Editor's Depot* there are some in there WAAAY better than the one shipped with the game. But oh well. Keep asking questions, you'll learn.
Good luck!
/Wolfrug out.