Well, we should then give up on any attempt to stop global warming to begin with; if there’s no anthropogenic global warming then our understanding of the climate and probably several laws of physics is severely lacking. We should also despite what the OP says assume that the polar ice will melt, because in this scenario we can have no idea how far it will go; our climatological models have just been proven nonsense.
As for what else we should do, building barriers against the ocean in low lying impoverished areas over the world like Bangladesh comes to mind. They can’t afford to themselves, I think, and moving that many refugees is likely to be a disaster in any number of ways. Assuming that they aren’t stopped at the border and left to die; any target country for them is likely to be unable to support them.
Stock up on seeds for various crops; there’s going to be a lot of replanting going on as land becomes unsuitable for one type of crop and more for another.
We should in general prepare for a worldwide famine and economic collapse; a worldwide combonation of the Dust Bowl and the Great Depression. How one prepares for a disaster of that magnitude, I don’t know. In fact, we should mostly act just like we should in the real world, because anthropogenic or not, not much is going to be done about global warming until it’s far too late. About the only personal recommendation I can give is don’t have children; we’ll have a hard enough time feeding the people already here, and the future isn’t likely to be a pleasant place to live.