Having players that are willing to spend time and effort outside of the gaming session preparing and planing is rare, do reward them. 

I think you should let the LOMC work spectacularly. It works exactly as planned, no side effect just bullseye and one hundred per cent success. Let the LOCM survive the initial use, and be ready to fire again and again and again. Now let them go conquer a kingdom or two. Or the entire world if your in a particular good mood.  

After a few initially successes let things start to slide, introduce the caveat of the story. Was there really no side effect of the initial blast? Spread rumours about mysterious things happening at the initial blast site. Are peasants suddenly manifesting special powers, medieval x-men? Is there a dangerous fauna growing out of the crater? Is there disease spreading? Did the the blast offend both angels and demons and have they declared a temporary cease fire to deal with this threats to the univers? 

What ever you do in the initial blast zone make it really bad. Make it so bad that they will have to go deal with it them selves, and give them a real hard time. You want them to crawl out of the crater after a few sessions thinking. "I hope this sh... is not going to happen alle the other places we used the LOMC."

Doing this you get two very distinct chapters to your campaign. You first get a chapter about them trying to conquer the world, and a second where they try to save the world form the mess they made while on the power-trip.