Answer: Make it appear as if the attack came from the Dust Brigade...OT image is intact.1) Absolutely destroyed image and the taint of it on their public image isn't going to help them any on the wider market. As a corporate entity, they aren't going to do anything that affects their image across any market with a gridfeed so negatively and hurt sales. And do you honestly think the other corporations wouldn't make a huge stink to hurt Omni's image and take away market share? And that Omni wouldn't take the same thing into consideration and dismiss the idea out of hand? Even if they could avoid blame, they'd still look careless and the situation could be spun into making them look extremely stupid, or extremely evil, neither of which is good for business.
Cheaper to terraform than pay thousands of employees and war machinery for decades to continue trying to take over such areas.2) The damage to the terraformed portions of Rubi-Ka would be monumental, costing the corporation untold billions, if not trillions of credits and probably close to as many wasted man-hours. Why would Omni, who has stated time and again that they are only concerned with the bottom line and the profits to be reaped, undertake an action that would cost them so much of that? Not to mention the disruption to the flow of Notum and the credits that trade brings to their coffers...
Point #1, Dust Brigade would be blamed...and the clans already have a big grudge against them...it would increase hostilities toward DB more than OT if they appear to be the guilty ones. Also such a wide area being nuked would certainly lead to chaos within the clans...the motivation you speak of would take place AFTER the survivors managed to regroup and organize, which would take time (enough time for OT to send in their elite to secure the areas they just cleared).3) The surviving members of the Clans would rally like never before, screaming for blood, and becoming far more formidable and ferocious with a huge motivation the likes of which Omni will likely never have encountered... Why give the clans such a huge rallying point? So many martyr's that could be pointed to? And why to a target whom they've dismissed on a few occassions as ineffective and meaningless when it comes to leadership of the Clans?
Because as crazy as Silverstone is (and he has raised his weapon against clan leaders once), he is not that stupid and the CoT and the Legacy clan leaders know it. He would never store such weapons in Tir, it would be a disaster and probably would lose the large support he has (not to mention his "valuable" Commanders currently stationed there). If he had such weaponry, it would be more likely to be stored near their HQ in Mort.4) Blaming the CoT for storing weapons of mass destruction under the council buildings sounds good, but why not just set one off in the city in the fair trade beneath where Silverstone likes to hang out and blame the Sentinels and use the same excuse? A lot easier and more likely an explanation if you're looking for plausible deniability in the matter, and a lot of clanners wouldn't doubt it to be true, so less damaging to Omni's reputation.
An EMP would destroy all electronics....yes, but it only takes ONE ship to deliver this strike, a cheap alternative to having to send dozens of warships and thousands of omni drones into a battle they may not win.5) Given the unpredictable and often windy conditions of Rubi-Ka, why endanger their own assets with fallout from such a blast unnecessarily? The EMP resulting from such a blast would damage Omni equipment, something we all know they value more than they do it's people if their treatment of miners is any indication, and that doesn't even begin to cover the radioactive crap covering most of the terraformed regions of Rubi-Ka.
Omni-InternOps would be the most likely behind such scenarios, AF and Unicorns are the muscle, certainly not the brains when it comes to OT warfare.