The Destiny trilogy involves a temporal causality loop and some very advanced aliens who somehow have never heard of Q or the Metrons. In the Star Trek universe, time travel/time cops and/or godlike aliens exist, and both have a reason to be involved in this scenario. It's really not that hard, after reading something like Watching the Clock, to imagine some purpose behind the coincidence that wipes the Borg from the timeline.
When did I ever state that the Caeliar hadn't heard of the Q Continuum or the Metrons? Maybe they'd encountered them and decided they didn't like them. We don't know.
Not really. The Q aren't known for playing well with others — and notice that the purpose of the "Great Work" changes at the end of the trilogy, in a way that might even put the Caeliar at odds with the sometimes meddlesome and fickle Q Continuum. Also remember that not all members of the Continuum behave like the Q who vexed Picard et al.
I'd say the Caeliar aren't known for playing well with others. By their own admission, they didn't get along with other species and went into hiding to avoid contact. And then you have Kintana, the third Caeliar city ship that went back to the Dawn of Time. Assuming they survived for a long-enough period, they would have a front row seat for the Universe As It Unfolded and would know who was worth knowing and who wasn't. And might I add that I find the whole Kintanta plot line to be a maddeningly unresolved one.
The ones who were flung back to the Beginning of Time (in the Kintana city-ship) are presumably the same ones who vacuumed all the space dust from several galaxies and built shells around the shells around the stars of one galaxy. The Enterprise-E visited that galaxy while exploring the Caeliar space tunnels; it's described in a chapter in Mere Mortals. They are also the ones presumably who caused the Cataclysm in Gods of Night which Inyx explained at the beginning of Mere Mortals. As to what exactly is up with his group, or whether the Cataclysm wave was just a stupendous piece of programming while the group itself is dead, that was never resolved in Destiny.
The timing of the Destiny events is a plot point in VOY: The Eternal Tide. Spoiler: VOY: The Eternal Tide (spoiler) The event was observed by the Q Continuum. It was indeed a furtuitous timing, completely accidental but vital for the entire galaxy. If any one of the principal ships had missed their part on the stage, the Borg would've assimilated the galaxy. Q states that Janeway's action in "Endgame" brought the Borg invasion about sooner - fortunately. Otherwise the Borg would've been strong enough to eliminate the Federation on a single day. It was part of why Janeway's death became a fixed point in time across the multiverse.
The Q and the other higher powers have their work cut out for them; we're beginning to see just how complex the multiverse can be. It's a wonderful web of apparent contradictions that actually all tie together wonderfully. Spoiler: The Eternal Tide, Watching the Clock, Q and A, and Destiny If the Borg aren't liberated in 2381, they'll grow too powerful to be stopped and will eventually assimilate the entire Milky Way by 2600 at the latest, and who knows where they'll go from there. Even before that, there's a fair chance that they'll wind up being the one to take the test established by Them, serving as representatives of the universe, and of course they'll fail utterly, causing the premature extinction of the current reality. Because of these factors, people in the know (those uptime or those on higher dimensional planes/other continuums) want to make sure the Borg-Federation encounters happen as they happened in the timelines where the Destiny events take place. Q ensures that the Borg and the Federation meet, so Picard and Janeway both are forewarned and able to fulfil their roles. He also gives Janeway navigational information to "shave a few years off her journey" and nudge her toward the transwarp hub, with Janeway of the alternate future then allowing present Janeway to cripple the Borg and provoke a full out attack a few years later - which is the key to stopping the Borg from passing the threshold of the unstoppable. Time travellers daren't mess with this because changing the events of 2381 leads to timelines where the Borg conquer the galaxy. However, because Janeway changed the past, the Omega Crisis remains unresolved and the universe is threatened that way, meaning there's need for a fix and Q loses his son, for which he hasn't forgiven himself while he pretends he's angry at Janeway. Have I missed something, because all that is pretty awesome.
Between my ego sucking up all the oxygen in the room and her talent crowding my words off the page, I don't think either of us would survive such an attempt.
I don't know if I would want that to happen. The amount of awesome that it would produce would be so great that nothing that came after it could ever outdo it, and thus all entertainment after it would become pointless and come to an end.