Assuming I have the technical means of redirecting sunlight from a position in interplanetary space (placing the mirror[s] wherever necessary), how much havoc can I wreak on the planet Venus? Of special interest to me: could I do so much damage that the beam burns away the atmosphere or causes physical damage to the structure of the planet itself?
You could disassemble Venus into her molecular components in short time.
I don't know Venus's gravitational binding energy, but it should be less than Earth's since she's less massive. Let's assume Earth's: 2.49 x 1032 joules.
The Sun's output is 3.86 x 1026 watts.
That means that with some very gross rounding, you could use a Dyson sphere to dissolve Venus in a million seconds. That's a little less than twelve days.
In between doing nothing and destroying venus you should be able to get the atmosphere so hot that individual molecules are faster than Venus's escape velocity, so yeah, you can remove the atmosphere. If your goal is to reduce the atmospheric mass to make Venus more terraformable, though, you may have to wait from millenia to millions of years for her to cool down after the feat.
How much sunlight can you focus?
Troy Rising postulates a solar-pumped "laser" that can push nearly 2000 petawatts, which reportedly can "destroy the Earth in 6 months, the biosphere in 16 days."
But that's boring. What if we tried more power?
Well, XKCD speculated what would happen if you aimed really powerful lasers at the moon. At a "measly" 5 petawatts, the surface turns to magma. At about 2.5 yottawatts (2.5e24 watts)... well, things get interesting. At that point, "ablative armor" comes into effect in a significant way. Moreover, the ejecta effectively becomes a propulsion system that pushes the celestial body out of orbit. Luna is smaller than Venus, but that just means your, ah, "engine" has more mass to push, but also a larger "fuel tank".
For perspective, that last number is about 0.5% of the sun's output. You're on your way to Kardashev II, but you've still got a couple orders of magnitude to go.
Pro Tip: If you want your planet to stay in place while you're obliterating it, make sure to hit it from at least two angles (the more, the better) and be careful to keep your forces from the ejecta you're producing balanced. (Note: this is true even for meteoroids, asteroids, etc., not just planets.)
Using enough sunlight, you could potentially knock Venus out of orbit (see: https://en.wikipedia.org/wiki/Yarkovsky_effect), even before you burn it up. If properly calculated, YOU could use it to destroy other planets by means of a planetary projectile.