Not saving Arabella isn't a game over screen. It gives you a solid reason to side with the tieflings in a major conflict against the druids.
These days, I feel like you practically have to try to let Arabella die. I don't remember the last time I wasn't able to save her.
I'll grant that the mind flayer scene was harsh, but it taught a valuable lesson, and let's face it, you're swimming in revivify scrolls. Which I never actually have to use, by the way, cause I can't remember the last time I had a character die in the game.
As for Nettie's poison, that's also not a game over moment. There are ways to neutralize the poison. That scene made Nettie an interesting character. She was willing to kill you and spark a serious conflict because of the tadpole. That scene had meaning and purpose in the game. Now, like the Arabella scene, you have to try to get her to poison you. You have to carefully navigate the options choosing the unlikely path that leads to that outcome.
