In seriousness, though, I thought the idea behind deflection wasn't so much the explosion pushing the rock, but the heat from the blast causing "venting" on the comet via vaporization of the ices. Essentially, man-made jets that would push the comet, just like they sometimes get naturally.
So how far into left field am I there?
This could make sense for comets, but that is not what is usually discussed. The most likely thing to hit Earth would be an asteroid, specifically a near-Earth asteroid (that is, one of the ones that orbits around the same distance from the Sun as the Earth - obviously any asteroid would have to be near at some point if it's going to hit us). Asteroids are made of rock and metal, so no venting. Any defense system would have to cover all possible dangers, so designing one specifically to work via venting, which wouldn't work on the most likely dangers, wouldn't be all that helpful.
Considering an ocean impact, preferably deep ocean, far from land, many small pieces would be far less destructive than one huge impact. Especially if they come in a line, so the earth spins and spreads out the impact zone.
They wouldn't be in a line. When people talk about asteroids as clouds of debris, they don't mean a huge cloud spread all over the place, they mean a collection of stuff held together by gravity, but without enough gravity to compact it all into a solid mass.
The problem in an impact is that the amount of energy is ridiculously big. It doesn't matter if it's one big rock or a collection of small rocks, when it hits, pretty much the whole thing, along with whatever it lands on, will be instantly vapourised when the kinetic energy is cnoverted to heat.
Incidentally, when you say "preferably far from land", that may actually be the worst possible case. An asteroid hitting land would make a big crater, wipe out anything in the vicinity and throw up a load of dust that would probably last for a year or two and would have an effect similar to, but probably more severe than, major volcanic erruptions. An asteroid hitting the sea would create tsunamis that would race around the entire world, and would release a massive amount of water into the atmosphere with major effects on the climate as well as disruption to the circulating currents which are an important factor in stabilising the climate. Obviously we can't be sure unless it actually happens, but most people working in this area consider an oceanic a much worse scenario than a land one.
Smaller pieces may not bury themselves deep through the crust, but shatter in the water, and just impact the crust, rather than blowing through into magma, leading to a planet killing event.
As far as I know, no-one has ever suggested an asteroid actually punching through the crust. Even some of the biggest impacts we've had, such as the KT boundary collision that got the dinosaurs, didn't even come close. There is debate about whether large impacts can influence tectonic activity due to the shock, and this certainly seems likely, but it's nothing to do with actually punching through to the mantle.
Many smaller tsunamis would be less destructive than one huge one.
Well, not necessarily. In any case, as explained above, the tsunami resulting from one large rock hitting the sea would be exactly the same as the tsunami resulting from a large, gravitationally bound collection of rocks all landing in the same place at the same time.
And there would be more atmospheric braking and energy transfer, many small pieces would present more surface area to heat and expend energy. Every square meter that burns is less to impact.
Firstly, as explained above, it's the energy transfer that's the whole problem in the first place. Secondly, there would be no difference in surface area or resistance between a single rock and a collection of rocks. What's important is not the total surface area, but the surface area in contact with the atmosphere. The front edge of the impacting body will push all air out of the way in a shockwave, so anything behind that won't have anything to offer it resistance anyway.
As a caveat - this all applies to large impacts on the scale of mass-extinction events. This would almost certainly be the case for a comet, but asteroids come in much smaller sizes. In a large impact, all that really matters is the total kinetic energy, but below a certain size, you're looking more at localised damage rather than global effects, and in that case the details become more important and some of your points have merit.
