I add sugar direct to bottles, always have.
I now leave my beer in the FV to finish and more besides. When the sugar goes in I might get a little foaming , but not much to cause concern. So yes that's the sugar nucleation thing.
However when I returned to brewing a few years ago I bottled as soon as the FG was achieved and I regularly used to get gushers, which I put down to the fact that even though the FG might be stable there may be more CO2 to come out of solution before the equilibrium is reached. And the remedy at the time was to leave it longer before I bottled, which I then remembered is what I had done previously to get round this problem, well before I found out about yeast cleaning up etc etc on this forum.
Finally if you allow the beer to finish then leave it then crash cool at the end, there will be less than the equilibrium CO2 in the cold beer at the time of bottling, and therefore less tendency for it to want to be displaced by the sugar as it is added.