I'm pretty sure it's something stupid. I'll freely admit I'm a complete newbie at scripting.
Regarding naming the pilot and Harrier, I basically copied most of it from your original script, except I added a couple of extra commands and changed the group name to "Alpha." So there could well be a stupid mistake here, but I can't find any reference in the Wiki as to how to set it up differently.
In the mission itself, the player is named s1 and I added Alpha= groupthis in the player's init line.
The script as it stands is as follows:
veh = createVehicle ["AV8B", position startpoint, [], 0, "FLY"];
"SoldierWPilot" createUnit [position veh, group Alpha,"this moveindriver veh; veh setdir 80; veh flyInHeight 100; doMove getPos s1;"];
~240
deleteVehicle veh;
exit
Since I can't even get this part right, I can't get any further with it.