This was, of course, an Exception of enormous magnitude and stakes, and after it happened, Civilization did direct quite a lot of its resources and collective intelligence toward managing the fallout and mitigating the damage. But there was only so much that could be done. Possibly the whole tragic series of events had been almost-guaranteed to play out like this from the point at which Mirrell decided to make a choice with predictable, and predicted, consequences.
(It's genuinely hard to know whether Civilization should have had a different policy of what to do, of how much to step in and meddle in the lives and decisions of two fully-informed adults. It's clear that Mirrell, making that decision, valued something a lot more than her own happiness, and maybe got it; she did express, strongly, that she had never regretted choosing to have Finnar, that she was grateful to have been a mother and glad that Finnar, specifically, existed, however much suffering those six years had brought her. And although it's not true that her choice harmed no one but herself and her equally-consenting-to-this-risk partner, because Finnar was very much harmed, it's still true that in the counterfactual, Finnar never existed – and though the actual Finnar who did exist was of course devastated and furious, it wasn't obviously the case that his existence was net-negative from his own perspective. Besides, while 'Mirrell is unhappy' was something the prediction markets saw coming, the resulting outcome was a lot worse than the median prediction.)
...Civilization does believe in causal attribution, whether or not it makes sense to frame it as blame. It has Official Experts and Standards about attributing causality (as plays a central role in impact markets). There was an answer, or at least a best available prediction, of "why did Mirrell truedie, and was anybody responsible for that even in the sense that they'd have been paid a bonus if that had been a good thing". Civilization also does not believe in concealing that information from involved parties, and there was a straightforward market-grounded-nonmarket-prediction of the counterfact that Miriel would've had a 30% chance of still being alive or in cryo if she hadn't given birth to Finnar.
That information was available to Finnar himself, if he chose to seek it out, which he did.
(From a fault analysis perspective, the main actor in the situation who made a deliberate choice, and could have made a different choice, was Mirrell, though the counterfactual world where Mirrell placed more of her trust in the policy markets, and was more willing to let that override what she wanted and cared about, does sort of boil down to "if Mirrell had had different values". And from another angle, one might attribute fault to all of Civilization, for failing to offer Mirrell something worth sticking around for. Just about nobody would place any fault, even fault-not-in-the-sense-of-blame, on the small child, who hadn't chosen to be born and could not meaningfully have decided to, instead, be a different small child who was easier for Mirrell to parent.)
From Finnar's perspective – a child more than smart enough to follow that causal analysis, but still with the emotional maturity and nuance of a six-year-old – it seemed pretty clear that his mother's death was his fault.
But not only his fault! Mirrell had, after all, cared about the wellbeing of her partner and her son, and stuck around as long as she believed her departure would leave them worse off. Which meant, from Finnar's point of view, that another blameworthy actor was the woman willing to remarry his father after his first wife's True Death. Or at least this was probably his reasoning; the observable effect was that, from the very beginning, Finnar hated her with the fury of a thousand supernovas.
(This was perhaps foreseeable in advance, if the right eyes had been on the situation before rather than after Mirrell's truicide. It's also unclear if it reflected a mistake in Civilization's priorities, that those eyes weren't on the situation until too late.)
In any case, Firrin wasn't going to subject his kid to a stepparent he detested, and so Firrin was not going to pursue a relationship with Indis as long as he was still raising Finnar, despite the fact that he was grieving, deeply miserable and clearly struggling to parent his traumatized and also very neurodivergent child (who on some emotional level he probably did resent after what had happened with Mirrell), and the prediction markets thought that he would be happier if he remarried.
Finnar was not oblivious to this, and was also very bitter and angry with his father. He was about nine when he started insisting that he would be FINE on his OWN and DIDN'T NEED A PARENT ANYWAY and everyone would really be better off if he MOVED OUT. Perhaps unsurprisingly given the sheer accumulated tangle of emotional history with his father, the policy markets agreed with this assessment.