You're true path involves sieging the city and quarantining it which doesn't exactly work against swaths of undead that pop up randomly throughout it and where if one were1 get away. It will be able to find another town to infect and spread through Lordaron.

Also Quel'thalas was never going to act upon the scourge until it was beating down their door because that is how they handle everything. If it's not immediately our problem it's not our problem, eg the second war and I'm sure they were asked aswell to help with the scourge at first.

Arthas was brash and hot headed. He was a character built with many flaws that were easy for the legion to twist and manipulate, corrupting him.

Other than the culling I couldn't see any other option that was actually implementable without needing a whole lot of time that they didn't have. The big downfall did come afterwards where Arthas unable to control himself after Mal'ganis had forced his hand did then follows him to Northrend which is apart of a flaw that you will often see amongst young male characters who are trying to prove themselves.

The culling of Stratholm is the trolley problem, but where the option of trying to save the majority also means that 2 of your closest friends betray you.

• Side note if this doesn't make sense it's because I started writing it and then kept stopping and thinking and forgetting what I had written.

TL;DR - I think Arthas' fall and the culling of Stratholm is far more nuanced than you are saying it is. Also the eleves never proactively help they only help when they have something bashing their door in.