Aethas may have extremely divided loyalties, but he does genuinely care about protecting people. He was seen working with Rommath during the Blood Knight quests in Shadowlands to confront a threat to Silvermoon, with no other Kirin Tor in sight. He also protected injured survivors in Dornogal, and was going without sleep after Dalaran fell because he feared losing more colleagues if he let his guard down to rest. He does care about Quel’Thalas and the Sin’dorei, not just Dalaran, so I could see him attempting to aid against a threat to the Sunwell, but I wouldn’t expect a major appearance since he’s likely to be busy with Kirin Tor stuff for awhile. Keep in mind that the Dalaran survivors include plenty of Sunreavers, and he’ll likely want to keep in touch with them.
With the new book announcement, I feel that Arator’s chances of getting past Midnight unscathed have decreased, though I don’t think he’ll die (permanently, anyway), especially since it seems like he might be a point-of-view character in the book. This is Warcraft, and dying isn’t the only bad thing that can happen to a character. There’s all sorts of curses or corruption a character can end up with, and there’s also just good, old fashioned maiming or scarring, as seen with Lor’themar.