you're not going to get that with widely dispersed efficient-life-killing thermonuclear strikes (leaving aside the obvious question 'what do you do about the vents and spores etc').
Where on earth did widely dispersed efficient-life-killing thermonuclear strikes come into it? Multi was considering a bomb (or cluster thereof) at a single location. The closest to 'dispersion' was when you brought Orion into it, with it's chain of bombs spread out over the launch distance.
A Project Orion kinetic strike would probably be more efficient than a pile of gigaton nukes since each explosion can be smaller and more energy extracted than it.
Or, alternately, it would be overwhelmingly inefficient because the projectile is aimed away from the planet - or at best along the surface of it.
I incidentally dispute your efficiency claim anyway. I'd be willing to bet that if you collect every one of the bombs you were using for your Orion weapon and place them in single location then it would be more capable of slippiting the planet than the projectile would have been. Even if you managed to make it target the earth directly. If necessary you would of course use several years worth of the entire earth's production of steel (and maybe lead, gold and anything else hard or heavy) and use it to cover the bomb and keep the energy around a tad longer.
Why does it need to be aim along the planet? Use orbital mechanics: Send your spacecraft on an orbit such that it hits the planet it launched from at the fast point of a very long elliptical orbit. Or even just at the far side of the current planet's orbit, whatever. It can't be that hard to get an impact at whatever angle you'd prefer with most of the Orion vehicle's energy, launching direction barely seems to matter.
http://neo.jpl.nasa.gov/risk/2012da14.html
http://rt.com/news/paint-asteroid-earth-nasa-767/
Seems like a good opportunity to bring up existential risks. And A friendly reminder that NASA is in fact pretty damned important.
Thoughts?