Aha, first comments and suggestions for you from having played around in the demo:
1) Remove the line
"map_misc",
from the mission.sqm file.
The script doesn't require the MAP_misc addon, so it shouldn't be required for the demo (did an old version of the script use MAP_misc invisible targets, perhaps?). When I first tried to run the demo is wouldn't work since I don't have the MAP_misc addon installed, but I just removed that line and the demo seemed to be worked fine.
2) There is nothing to stop me spamming artillery requests on the map. I managed to grind the game to a virtual halt by requesting artillery about 20 times in one second. May be a known issue, but as it wasn't mentioned in the readme, I thought I'd mention it here. When I waited for all the fire missions to complete, the game seemed to keep being slow, so I wonder if the scripts aren't terminating properly? (I was still getting a tenth my normal FPS a couple of minutes after the last shell explosion/chat message).
3) I'm not sure if this would work, but if you are limited to 5km range because the shells time-out, have you considered deleting and recreating the shells every 4km? Again, I'll state that I don't know whether this is feasible, but someone else might be able to say so.
Don't think I'm being negative; I think it is great that someone is modelling real physics for deviation rather than the normal "_x = _x + (random30) - (random 30)" dispersion technique ;P Good luck with this in the future!