On the ethics of Rowan killing himself, his assumption probably is that Alexia would then be raped, tortured and killed and not necessarily in that order. If Rowan was willing to discard Alexia's life, he has far better options than killing himself. He can betray the twins at several points and lead to their destruction. Sure they'd use the amulet to kill him (and do the above to Alexia) but he could really raise the alarm and destroy their chances.
I agree wholeheartedly with what you said.
I do think there is nuance in regards to Rowan having a " hero complex" instead of just straight up being a hero though. It seems that Rowan can still maintain the lie of being a hero to himself as long as he is able to protect a few people who are close to him- he might not actually be a hero in that case- but he still thinks he is.
Existentially, we constantly see Rowan having to make choices about being heroic or not and the twins tend not to give him many options- but Jez usually dangles one little shred of hope at the end of committing atrocities to allow him to have a small "heroic" act. After taking the keep he can "save" Hel. After Rasteld he can "save" Juliette. She pushes Rowan to try to start relations to "protect" Draith. Liurial could also be a Jezera move- we don't know about that.
If we view this sort of hero complex as Rowan's Achilles heel, which I'm sure Jez would after how successful using Alexia was for the same purpose then it becomes pretty obvious why there would be an interesting and nuanced reasoning for Jezera to try to make a Rowan harem. Specifically of weak characters who don't really have the ability to help Rowan fight back in any tangible way (Juliette is a good example of that). His hero complex is maintained- but he will still be willing to do horrible things to try to protect them.
If you follow Alexia's NTR pathing it makes even more sense since Jez needs to maintain some influence over Rowan. If he ever found out about the Andras NTR pathing in particular why the fuck would he be sticking around Bloodmean. Jezera's NTR pathing is much more in line with her long term goals of keeping influence over Rowan- but it too could be discovered and it'd be hard to predict how he'd take it. So she needs insurance to keep that influence particularly if Alexia is on Andra's route.
Since you know....Andras' route doesn't help the twins with their long term goals at all. Andras NTR is basically dumb-ass red-demon with daddy issues wants to steal someone that is with one of the heroes that killed his daddy because he is an edge-lord child. For me Andras is the most boring villain in the story- Jezera carries the villain plot. Removing him wouldn't subtract from the narrative at all- he could basically be replaced by any orc in the castle....just slap some tattoos on the green fucker first.
I think its ludicrous to assume the VN authors would be unable to write harem kink pathing into the narrative compellingly-they already laid compelling ground-work for it and they are brilliant writers. The primary problem with other harem games isn't harem itself- it is poor writing (which is the problem with most porn games to be perfectly fucking honest).