At some point you have to choose what to do, and that's always going to involve unknowns. For instance: 40 years ago, the USA could have chosen to continue building out nuclear power. Not accelerate the deployment, mind you, merely continue the roll out apace. If we had done so, we would be completely free of coal and gas today. Not maybe 30 years from now if we hustle, today. One could rage all day against the risks of another meltdown that this would entail, but when one considers that the "reasonable cool-headed alternative" we chose instead was to pump the atmosphere full of CO2, suddenly that doesn't seem too bad.
It's easy to rant about the negatives of any given tradeoff, but for the rant to be meaningful, it has to be compared to the alternatives, which in this case are "do nothing, adapt to climate change" and "spend truly mind-boggling sums of money to accelerate renewable rollout." Neither of those is good. I'd personally prefer to spend the money, but I don't get to make this choice, and since "do nothing, wait and adapt" seems to be the default alternative, and it's chock-full of unknowns, it's hard for me to say that the geoengineers are significantly worse, even though their risk profile is, as you point out, steep.
Potentially destroying the ocean vs spending money to reduce carbon emissions is a no brainer, and it's the job of these scientists to say that out loudly rather than to push for these extreme solution which can produce a blackswan disaster of epic proportions that even money won't be able to save.
The arrogance of scientists to "try out my method instead" is at fault since if these scientists were to actually want to save the planet they would push on politicians to spend money rather to try and implement potentialy extremely destructive solutions.
In short, you fail to understand that this might create a unfixable disaster, while global warming is obviously solvable by other means since we are far from any kind of an serious crisis, regardless of what "scientists" say.
The world is NOT burning yet, which means we can and HAVE TO avoid stupid solutions like dumping iron in the ocean while there are harmless solutions available.
Scientists have been pushing on politicians to lower emissions at least since the early nineties. There is no discernible dent in the rise of CO2 levels. There is not even a big dent in the rise of emissions: Not even the second derivative is pointing in the right direction.
