Spock was the main inspiration for reunification. It had to be before the 32nd century and (in my opinion) after the temporal cold war.

After the destruction of Romulus, the Romulans were scattered across the galaxy. Of course they would take the opportunity to return to the planet of their origin.

There have been no set dates for when Ni'Var came about as of yet, and I think Trek's in no hurry to change that just yet, given the vast amount of time between early 25th century and mid-32nd century in which it presumably took place. All we know is that it happened sometime prior to the 32nd century (probably before the Burn as well).

As for the Romulans choosing to live on Vulcan...why not? It's their world of origin, and as we know, Romulus is gone now, so they were already effectively homeworldless otherwise.

Would be a great story to tell @ some point.

I am not sure the date has ever been announced