We don't understand enough to ensure anything we've done wouldn't cause unintended negative side effects but as it stands we're headed for utter destruction so doing something might be better than nothing.
No. Doing wrong things can be much worse than doing nothing. If you kill off too much biosphere for example, there goes humanity.
Volcanos tend to throw a lot of sulfur in the air which is deadly.
We could disperse dust in other ways, but the risk is that we'd increase albedo of Earth and cook it even more.
I read that one possibility is less rain. Which is obviously something we don't want. It's already not enough.