A couple of additional points:1. If the sunshade were rotating and anchored to a large asteroid with a tether they could balance each other at the Venus-Sun L1 point, with the sunshade lowered towards the planet it wouldn't need to be as large as it would if it were at L1 or sun ward of L1 (to counter act the light pressure from the Sun). The asteroid anchor would be sun ward of L1.2. The H2SO4 clouds on Venus are a huge inhibitor to the rate of cooling, but these clouds are a product of a photochemical reaction:http://en.wikipedia.org/wiki/Sulfuric_acid#Reactions_with_non-metals Sulfuric acid is produced in the upper atmosphere of Venus by the Sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapor. Ultraviolet photons of wavelengths less than 169 nm can photo-dissociate carbon dioxide into carbon monoxide and atomic oxygen. Atomic oxygen is highly reactive. When it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapor, another trace component of Venus's atmosphere, to yield sulfuric acid. In the upper, cooler portions of Venus's atmosphere, sulfuric acid exists as a liquid, and thick sulfuric acid clouds completely obscure the planet's surface when viewed from above. The main cloud layer extends from 45–70 km above the planet's surface, with thinner hazes extending as low as 30 km and as high as 90 km above the surface. The permanent Venusian clouds produce a concentrated acid rain, as the clouds in the atmosphere of Earth produce water rain. The atmosphere exhibits a sulfuric acid cycle. As sulfuric acid rain droplets fall down through the hotter layers of the atmosphere's temperature gradient, they are heated up and release water vapor, becoming more and more concentrated. When they reach temperatures above 300 °C, sulfuric acid begins to decompose into sulfur trioxide and water, both in the gas phase. Sulfur trioxide is highly reactive and dissociates into sulfur dioxide and atomic oxygen, which oxidizes traces of carbon monoxide to form carbon dioxide. Sulfur dioxide and water vapor rise on convection currents from the mid-level atmospheric layers to higher altitudes, where they will be transformed again into sulfuric acid, and the cycle repeats.So once a sunshade was in place and the clouds dispersed heat in the lower atmosphere could radiate directly to space on IR wavelengths not blocked by the atmosphere GH gases.
Actually the principal inhibitor to heat transmission is the thick CO2 atmosphere that is opaque to much of the IR radiation. The air velocity is so slow at the surface that radiative cooling is the only mechanism for heat transfer. You have to stir things up at the surface to encourage convective cooling as well.
(better later than never)I know the CO2 is the principle inhibitor of heat escaping, the point I'm making is that the Sulfuric acid clouds are an atmospheric component that may be easily dispersed. Stirring up the atmosphere does nothing in itself to increase the rate of cooling because with the adiabatic lapse rate the atmospheric structure is stable
It would take hundreds of years for the atmospheric O2 levels to get high enough for a breathable atmosphere, if we rely on the natural buildup from plants, but perhaps this wouldn't be too much of a showstopper to habitation, most of the population could live in domed cities, or even if O2 levels were only high enough inside building for natural respiration, small implanted liquid oxygen canisters, feeding the heated O2 directly to the lungs that activated automatically when inhaled air was too low in oxygen could allow very normal living in a near pure N2 atmosphere, the only thing the person thus equipped would notice is that they exhaled 25% more volume than they inhaled.