Even trickier would be finding the right bacteria that could survive in that hostile environment.
When you are done using the asteroid for "nudging" Venus around, you park in a rather low orbit around Venus, and voila! You have a moon to generate some tidal action.
The math for this is fascinating. What would be the required mass and velocity to move Venus to a sustainable, higher orbit? What would be the ramifications of moving the planet to a higher orbit, but not increasing the speed of orbit? Would Venus end up in too eratic an orbit to sustain its place in the solar system? What would that do the remaining inner planets?
Well, 3 things are possible here:
(a) For small colonies, we create artificial magnetic fields locally, to protect the folks from radiation.
Talk about power generation! How many Gigawatts would that take?
(b) Since we have "spun up" Venus to a 24 hour (or so) rotational speed, we might find that the core of the planet will start generating a magnetic field like the Earth.
Number and size of comets required??? The number of trips to the Kuiper Belt would exhaust the resources of Earth. Or, would we just wait for strays???
(c) If the "moon" we park around the planet is sufficiently massive, tidal effects might help start some "core" sloshing around in Venus, to stimulate magnetic field production.
We would need some new oceans. But then, if we chose the right bacteria, oxygen and hydrogen creation could be the proper by-product. Any of those little bugs floating around???
One thing you need to keep in mind regarding any ideas of terraforming Venus is that Sagan’s “super greenhouse(TM)” theory is a bunch of bullshit. Venus is basically hot because it’s a fairly new planet which simply hasn’t had time to cool off. There’s likely nothing we could possibly do that would cool it in any sort of a hurry.