> I find it difficult to believe that no one has already done it
At this point there is no need to. The population that matters most politically (so-called "first world countries") are not affected by global warming enough to require drastic active measures. There is not enough political capital to be won in leading international talks on active Earth cooling. However, if the pain become real enough, talks will be held, rockets will be launched and the Earth will be cooled.
> Greenpeace would already be doing it
First, AFAIK Greenpeace is mostly against active measures. Second, "cheap and easy" is relative: it's "cheap and easy" as in "building a dam", not as in "occupying an oil rig".
[0]: https://en.wikipedia.org/wiki/Newman,_Western_Australia
Where do you think their food comes from, for example?
Presumably you're referring to Aboriginal Australians and Inuit/Eskimo peoples?
It might be useful to note that these groups of people are known for their utmost respect toward nature, and the fact that their communities are largely sustenance based.
> Third, it's possible to drop global temperatures in a few months' time if direly needed. Ranging from orbital sunshades to tropospheric aerosols, solutions can be cheap and easy to deploy.
If this were truly the solution to global warming, I find it difficult to believe that no one has already done it.
Saying "can be cheap and easy to deploy" would lead me to think of this were actually true, Greenpeace would already be doing it.
I believe that your explanations don't stand up to overwhelming evidence to the contrary. Climate change is accelerating, and it is a big deal for our and future generations.