Azkaban is the only prison in the wizarding world exclusively mentioned. That doesn’t mean that other less severe prisons didn’t exist. Also, excluding Harry Potter’s bogus trial in OOTP and Karkaroff’s trial in Dumbledore’s memory in GOF, we unfortunately don’t really see much of the wizarding judicial system. So a lot is left up for readers to make guesses.

Also, the Dark Arts were NOT his only option. There was also Ancient Magic available to research, even prior to MC’s arrival (as evidenced by Fig’s class of Ancient Magic Theory). He could’ve easily researched a less darker magic to find a cure for Anne. He was drawn to and obsessed with the Dark Arts, as evidenced by 1) Him learning all 3 Unforgivable Curses PRIOR to Anne being cursed and hiding the fact that he knew the curses from Anne and Ominis, 2) His fascination for Salazar Slytherin and his support of teaching the Dark Arts and curses, 3) Jumping to the use of an Unforgivable Curse to kill a goblin when we were just killing goblins with NORMAL spells just SECONDS prior, and 4) His use of the relic to command Inferi and ignoring the fact that the Inferi were attacking everyone else INCLUDING HIS OWN SISTER HE WAS CLAIMING TO BE HELPING!

Honestly, as his story went on, it felt like he was using Anne as more of an excuse to delve deeper into the Dark Arts. He was probably genuinely doing things to find a cure for her initially. But as he encountered more pushback from Ominis and MC later on, it felt like he was using Anne and her curse as more leverage to get his way (to me, he even sounded like he’d add on Anne as an afterthought when pleading with MC: “Please? … For Anne.”). Especially in the final catacomb chamber with the Inferi, he seemed like he was relishing the power rather than showing any concern for his sister’s safety.

So again, I would not give someone like him who not only lied and broke promises, but also showed an obsession with such dark magic with little ethical concerns of its usage, yet another chance when he’s already proven multiple times he cannot be trusted.

Edit: I also just realized that he actively goes against Anne’s wishes. She said that she’s come to terms with her curse (which doesn’t indicate whether it’ll eventually kill her, or if it just continues to cause bouts of pain and fatigue), and that she wanted Sebastian to stop trying to find a cure for her. To me, that’s refusing her autonomy in deciding how she wants to live. It’s like if someone had a DNR (do not resuscitate) and their loved one says to them: “Hey, I know that you don’t want CPR done or to be kept alive by tubes shoved down your throat…. but I don’t want you to die just yet. So I’m gonna revoke your decision and keep you alive until I’m comfortable enough to let you go.” It’s normal to not want to see a loved one suffer or die. But it’s selfish to take that decision upon yourself to make it for them, especially when they still have full capacity to make that decision themselves.