Everything being equal, the force needed to bust open the cap is equivalent to the force that it is containing, plus some. Not to get too technical, and indeed I am NOT a volcanologist, but the plug is likely many hundreds of meters deep and would require a small nuke to really crack it open (until it’s ready to crack on it’s own… the 1980 blast erupted clocked in at 24 megatons of energy, which was, we conclude, the burst point for the cap at that time.) Imagine trying to get authority to use a nuke past civilian powers-that-be…
One question: Didn’t any of these guys see “Armageddon” with Bruce Willis. You drill a hole really deep and then you drop the ordinance. So simple!