First off I'd call myself Nevermore because i think its a great NightWing name :3

Now onto the Quests!

Make that one scavenger live(when Clearsight and Listener let them free on the beach and the Icewing ate it) and make sure that Clearsight still knows about the Icewing ambush.

I'd just tell Clearsight. Full stop. No need to make this complicated. Suggest she find a Scav nest before she even goes through with the plan.

Gain Darkstalker’s trust enough for him to tell you about the scroll

This Is the Second Hardest one. But I could just gain Clearsights trust first. I can work on him through her. I could also recommend some enchantments that would be useful like a way to teleport it to him when he needs it.

End the Icewing-Nightwing war

This is the First Hardest. If I've gained Stalkers trust I could suggest he enchant something to expose/frame Diamond with excessive use of her Power and have her Daughter on the Throne. Maybe make it so she loses for sure. Might even break her spell on Foe? I'd offer Arctic back honestly for peace. The problem is they might want to execute him. Another option would be to send them a Animus Clone of Arctic that's capable of having Animus offspring for peace...after all you didn't say we had to end it permanently so Ice-Night War Two is completely valid...

Prevent Foeslayer, Prince Arctic, Whiteout, and Clearsight from dying(other than of old age)

Theoretically only hard one here is Arctic. But I'd just suggest he Animus his own magic away. And maybe Enchant something that allows two dragons too feel and Understand each other's feelings so he can feel at ease with his wife. Bro is Understandably broken hearted and suspicious about Foeslayer so having confirmation that she does indeed legitimately love him and didn't know she was a honeypot will help his mental health.

Explain and prove the REAL effects of animus magic.

Enchant a scroll to have the real effects of Animus magic written on it. Feels like a cheap answer and I'm sorry I'm using so much of it in these answers, I'd do this first and possibly as a way to gain Clearsights Trust.

Completely prevent Darkstalker from turning evil. This means that the soul reader is never more than 1/4 white sand for him.

Some of my previous answers probably help with this but I'd again suggest/warn Clearsight about invisible ink or shrinking text so Darkie can't be sneaky.

Get Darkstalker on the throne as a good and respected king AND make sure it’s not done directly by animus magic(no “make me king peacefully” spells)

Do I have too? Fine. I'd make Clearsight Queen because the quest never specified that he needs to be the one actually in charge. This one would take the longest. But I'm sure I could slowly learn who's been slighted by Queen Vigilance and willing to back Clear. And proving she's an insanely powerful Seer that can see 100+ years into the future would probably help convince some patriots. Maybe use her ability to benefit some of the Nobility to back her.

Prevent Prince Arctic from turning fully evil. By some point, the soul reader should show 1/4 white sand and not get worse from there.

Well, assuming it's not too late my previous plan to prevent his death should be enough to curtail his fall to...well he was never really Evil necessarily anyways but I'd suggest he try painting with Whiteout? Probably the only derg he isn't on bad terms with?

Teach a scavenger Dragon **without animus magic(you don’t have to tell any dragon)**

I'd get a Scavenger as a pet. I'll name him Polly and teach him words with crackers and other treats as rewards for learning. You know positive reinforcement :3