Dislocation of tens of millions of people is a bigger deal than most people think. It's the stuff that brings down nations and triggers wars and famines.
It is, and it could approach hundreds of millions. But there are a lot of other risks, including the uncertainty of a possible carbon explosion, possible unrecognized dependencies on marine life that's likely to experience massive extinction, changes in food production capacity, loss of access to water, more severe weather, and deadly heat. We really don't know what the ceiling is. It's almost certainly lower than Venus...but how much lower?