Would impacting the planet with comets at the right angle (tangent) give it enough spin to turn its iron core, kickstarting a magnetic field while bringing enough water to it to reduce the amount of carbon in its atmosphere?
Magnetic fields don't appear on planets just from spinning. It doesn't seem to be well known how planets get magnetic fields. Juno will explore this presently. At Jupiter only Ganymede, the Jovian (=large) moon with the next to the longest period of orbit = rotation as they are tidally locked, has a significant magnetic field. Mercury takes 2,800 hours to spin, but it has a magnetic field. Mars spins at almost exactly the same rate as Earth, but it's magnetic field strength at the surface is about 20-30 times weaker than that of Earth's and insufficient for its mass, composition and distance to the Sun to keep a substantial atmosphere.
And why would you need a magnetic field? There's no correlation between atmosphere and magnetic field. See Mercury and Ganymede (magnetic fields but no atmospheres), Titan and Venus (atmospheres but no magnetic fields) for the black sheep in that assumption. For radiation shielding, either helps.
Maybe a Pluto sized comet hitting Venus could disintegrate all of it and reassemble it in a way so that the new planet gets a magnetic field and a thinner atmosphere, and why not a moon. But that's well beyond engineering. Maybe a giant comet impact made Venus the way it is today, resurfacing it and bringing volatiles to create its thick CO2 atmosphere. Comets carry a fair share of carbon, so I doubt that a tangential impact would remove any of it.