trigger1 setpos getpos trigger1 + random # < trigger1 radius=15m
Randomizing the target will force the mortars to range in each time, so it's going to slow down the whole process.
A quick solution might be to use the fired event to grab each round as it's fired and randomize it's velocity slightly.
Sorry I cant offer an updated version just yet, these scripts have been gathering dust for over a year. At a push you can override the addons scripts and add a random element directly to FCRangeMortars.sqs. I can post deatails if you prefer that?
Or create your own version of the Mortar addon your using and add more dispersion.
BTW What Mod are you using this with? I have the original scripts used to create the addons. So if you dont need the AI Observer, you could use those scripts. There not in a pbo so they should be easy to modify for your needs.