Well, once the cap is open, it'll flow. The power behind volcanic eruptions is the exact same thing that makes soda fizz when you crack the cap on a 2 liter of soda : dissolved gas escaping under considerably less pressure than was previously present. So, if you crack the cap, the lava will "fizz"; how violently it fizzes depends on how viscous the lava is. The thicker the stuff is the more gas it holds and the more explosively it's released. Hawaii lava is thin, which is why Hawaiian eruptions tend to be calmer than most. Thicker lava, bigger eruption.
I don't think you could crack the caldera open with a single nuke; not even a 20 megaton kabanger placed 2km down. The caldera floor (ceiling of the lava chamber) is 5 miles thick and dozens of square miles in area.
But the really BIG problem that's being overlooked is this : there might not even be enough pressure in the magma chamber to start an eruption right now.
As for the plume of lava as high as Mt Everest. No. An ash column as much as 25 miles or more high? Yes.
There's a really good article on the Yellowstone volcano here : http://www.bbc.co.uk/science/horizon/1999/supervolcanoes.shtml
complete with transcript of the show.
There was a followup with more information and scientific background (I hated the whole "superdisaster" drama aspect of it) that aired recently. I find Volcanoes fascinating and supervolcanoes especially interesting.