How many comets would you have to crash and how would one redirect and crash them that wouldn't make this exotic-level?
Depending on a size, probably on the order of tens of thousand.
Comets in the Oort cloud take very little energy to put on a collision orbit, the Sun barely holds them gravitationally, orbital speeds in the Oort cloud are measured in _meters_ per second. So they require (relatively) little energy to put them into a required orbit. It might be doable with just regular thermonuclear charges.
It will then take these comets more than a thousand years to "fall" from the Oort cloud.
We'll be dead by then I'm afraid.
My understanding is that it was water that started the whole thing. water vapor is about ten times as effective a greenhouse gas as co2. and the runaway heat started with too much water vapor(closer to to the sun I think), leading to more heat, leading to more water vapor. eventually it got hot enough to start liberating co2 out of the rocks. leading to the current situation today where venus has way too much atmosphere.
Which is to say, putting a ring around venus to block the sun may have merit, but adding more water sounds like pouring petrol on a fire.
1. "Humidify" the atmosphere by crashing comets into Venus. This will also allow us to create a temporary "cloud" around Venus that can shield it from the Sun and lower down the temperature.
2. Once the temperature is low enough, Venus will get oceans on its surface.
3. At this point, CO2 can be split into carbon and oxygen. Oxygen will be immediately bound by the huge amount of under-oxidized iron on the surface, and carbon can be buried under the new ocean. Essentially, carboniferous age for Venus.
4. Once this is done, the atmosphere will be mostly nitrogen (at ~3 bar) and people could live there with just respirators. Eventually, once the surface iron is oxidized, the atmosphere can even be made breathable.
Apparently, this can be done within 2000-5000 years without any exotic-level engineering.