I think it's supposed to be either 15 years after the end of either the show or Nemesis, not from Hobus itself.
Picard's advice given to the New Atlantis Project turned out to be disastrously wrong. Uncorking the world's oceans back into the mantle has so far lowered sea levels by 100 meters while starfleet ships use weather control to battle the rising clouds of steam in a hopeless neverending battle to save the planet. Picard even has to irrigate his vinyard. Mon dieu!
Dialogue from the trailer said "Fifteen years ago". Assuming the event being talked about is Hobus, the current year should be 2402.
1) "Fifteen years ago, you led us out of the darkness." 2) "You commanded the largest rescue armada in history." 3) "Then - the unimaginable." Might be three distinct events in Picard's history. Fifteen years from the day Picard gave Romulus good political advice or whatever; X years from the day he commanded an armada; Y years from the day the unimaginable happened, possibly with X=Y. The unimaginable would no doubt be the loss of Romulus. The armada could refer to this event, but there might have been a rescue before that, too - say, Picard helping dissident Romulans emigrate to Vulcan or something. "Leading out of darkness" is an odd way of putting it if Picard just gave this bunch a lift from a doomed planet that was not particularly dark in itself (and was only to get brighter with the events, albeit very briefly!). Timo Saloniemi
1) Fifteen years ago you led us out of the darkness. 2) Then -- You commanded the largest rescue armada in history. 3) Then - - The unimaginable. If you care to include the Countdown comics, by the time of Romulas' collapse Picard was already out of Starfleet and working as a Federation ambassador. Maybe all three events happen prior to Romulas collapse.
The only thing anyone's said is variations on "twenty years later" though Patrick Stewart did make a reference to "nineteen years have gone by" though it's unclear if he meant in universe or in the real world. If it is indeed 2402, that's twenty-three years after 2379 and fits a vague definition of "twenty years" after Nemesis.
Something happened to the rescue fleet. Something had to happen that he blames himself for. Something that he at least thought of as an error in judgement. The "unimaginable" cant simply be that Romulus was destroyed. That wasnt his fault. And why would that cause him to quit Starfleet? Maybe the rescue armada was wiped out. Maybe he didnt order them to withdraw quickly enough. IDK. Something he blames himself for. A catastrophic miscalculation.
Yes, possibly. But for dramatic purposes it would be better if it was something he had done. He's the central character of the show and it would be better if there was something he was struggling with. If it's "hey I did everything right, but those f'ing Admirals f'ed up", that shifts blame and burden to characters we dont know and may not ever even meet.
"US" in this is probably Romulans. Romulus exploded, dark times. Picard to the rescue. He creates a rescue armada to try to save as many romulans floating in space as possible. But then the unimaginable.... Yeah, the horror, the horror!
The "we" could be a reference to Starfleet, ie Starfleet let Picard down. Maybe Picard asked Starfleet to act faster but they dragged their feet. So Picard gathered the rescue fleet himself when he saw that Starfleet was too slow to act, but it was too late. Maybe it is a bit more sinister and we learn that elements in Starfleet actually played a role in the destruction of Romulus. This disgusts Picard. I think it would have be something like this where Starfleet essentially betrays Picard in some way, that would cause Picard to leave.