When I first saw the film, I assumed that Nero had a way to travel through time at will, but with no particular precision as to when he arrives. That would have solved the 25 years problem and would also mean the Klingons don't get 25 years to study all that future tech (while still of course leaving the ship intact and stealable for Nero's escape
). TBH it's only recently I've learned otherwise. If the actual film explanation (deleted scenes aren't canon IMO) is that he just hung around for 25 years, then I think I prefer my initial assumption.
Still seems weird that he wouldn't try to warn Romulus, though. I suppose the Federation probably will.
But on the issue of destroying the Narada:
This is Captain James T. Kirk of the USS Enterprise. Your ship is compromised - too close to the singularity to survive without assistance, which we are willing to provide.
There doesn't seem to be any doubt in Kirk's mind that the Narada is utterly boned. Ethically, since we're told the Narada's destruction is certain, I see no significant difference between watching it happen and helping it along. Beaming off other Romulans who may not share Nero's appetite for destruction - well, they'd have to drop their shields, wouldn't they? Sounds risky.
But of course the real reason they pull the trigger is that it's cooler that way. With no ethical barrier, I've no issue with it at all.