Cazador
If every decision was accompanied with perfect information, then there wouldn't be much to the decision-making part of the game, would it? Might as well have the game highlight every good decision in blue text and highlight every bad decision in red text and call it a day, now the game gave you all the information anyone could need to make the best decisions.
I don't really know False Hydra, but I think comparing a videogame to a table-top campaign is a bit apples to oranges when it comes to the design, one has the figure of the DM and the other doesn't, what I took from that quote was that the person who made that campaign thinks a DM shouldn't automatically shut down questions from the players with "your character shouldn't know this", but a videogame dev can't really take questions from the players and "patch" the game on the fly in the same way a DM can.
You keep talking about lessons the game should have been teaching you but it feels like you are ignoring said lessons and taking odd conclusions, how exactly did you conclude that the game taught you that "haremettes can't die"? Most average saves at this point had scenes like Hilstara being grievously injured, Robin having close calls when she had to face against the Fucklord, and probably losing at least one of the unique orc allies. I have no idea how a player that went through those things would go into chapter 5, where the stakes are higher than ever, and feel cocky that "no way any of the women could die though", unless they are not respecting the game and feeling cocky for meta reasons.
But regarding the specific issue of Trin's possible death, I said someone can look a guide IF they fall into the fringe scenario of a player liking Trin but still managing to fail her check, which to me still feels like a very unusual scenario and I will elaborate on it element by element in a bit, but first I just wanted to highlight that no playthough is doomed to lose Trin, the divine shard and the Ravaging Conqueror scene means that even in the worst case scenario the player only has to replay the start of chapter 5.
First the RP point, as I said before, the overall result of the big events is not particularly relevant, most of Trin's points comes ruthless decisions and using her for copies. You said that players might not do any of those things for roleplay reasons like ethical concerns, but if the player's stance is that they do not condone Trin's ruthlessness and cruelty in any way, then why do you think it will be such a big deal for those players if Trin dies? I'm not saying that most players' reaction will be "good, fuck her", her death can still be dramatic and make the player a bit upset, but I doubt it is gonna be "oh no, not my dear and precious Trin, how could this happen to her, this playthrough is ruined" levels of upset. The whole point of a good relationship with Trin is that Simon can meet her in the middle and turn her "evil" into a tool for "good", if the player is not willing to make use of her even as a tool, her character doesn't really have many redeeming traits and better people have already died in the story.
The Ravaging Conqueror scene, you are simultaneously treating it as a "random scene" that shouldn't have too much weight but also saying the player could refuse it because it happens after Tanurak had just tried to recruit Trin, but that's exactly what gives the decision meaning as a test of trust. Without the Tanurak scene, I would agree with you that the Ravaging Conqueror scene being important to keep Trin alive would be complete bullshit, but the fact it happens imediatelly after the Tanurak scene should have the player questioning how they really feel about Trin, if they trust her or not, if the player can't trust Trin even at this point then it doesn't make sense to assume the player will feel that Trin is "too precious" of a harem member and shouldn't be allowed to die.
Finally, the divine shard, I will admit that this relies on the player holding on to a divine shard "just in case", which I think is good practice but the game doesn't necessarily encourage you to do so. But lets assume a savvy player held on to a couple shards in case they could be used reactively. The player saw that Trin was being courted by Tanurak, and they don't really know if they need to go a step further to protect her, or to show her that they trust her and deepen their bonds, well gifting her a divine shard sounds like something that would safeguard her, assuming that the player does trust Trin.
None of those things really feel random or esoteric, they are all tests of how much the player appreciates and trusts Trin, considering she would probably be one of the most polarizing characters in the harem, it makes sense that her dying or not is a reflection of how much the player was willing to trust and care for her. Maybe Sierra is being a bit too strict by not accepting more than 1 failure on those requirements, but none of them are particularly inaccessible, so my final point still stands that people that like Trin a lot will probably not lose her even in a blind run, but if they do, then quickly checking a guide will show they only need to replay at most chapter 5 to be able to save her.