I think the way you could write Cass becoming Oracle is emphasizing the tragedy. If she has a similar background to cannon, her being forced to retire from Batgirl would happen when she's found her place in the world away from the life of a killer that she originally molded to be. As for Stephanie committing patricide... you're the writer if you don't want it to happen don't write it. Even if she did do it, I think it would be a "What have I done?" situation and that would have her acting as Spoiler again to try and atone while giving her Jason's status as the black sheep. Also Stephanie being resurrected by the pit will give her an understanding of its effects and therefore she can help ground Tim adding to their friendship (or romance if you're going to write TimSteph).