So, let’s talk about what you are reading into things.
“And a deus ex machina powerup to win the day” does not equal 1 shot.
“The whole ‘power up to match the enemy’ trope” does not equal 1 shot.
So, again: Nobody said anything about a 1 shot.
I think this is because you have not really thought through the problems with the trope in a continuing story.
Stop and really think this through. Think about what power Xal’atath currently has. She is wielding the power of a void lord. If you empower any single hero character to even close to that level then every single problem going forward has to be more powerful than a void lord, or it is trivial. This ends up in a cycle of escalation to the point it eclipses every other story. It all becomes bland.
The trope of a hero rising up and becomes god tier super strong can work (if it fits the character and narrative) when it is a self contained story. Character takes on god tier power, saves the world, curtain call, story ends. But it becomes extremely problematic when the story has to continue with god level powers being available.
And that is before we even talk about Liandrin’s arc and the well itself. There is not an interesting story with the Sunwell just empowering her. It doesn’t help her overall arc. What you are suggesting is nothing more than a deus ex machina ‘here is your power, go win’ story. That would be a bad story and do a disservice to Liandrin’s character arc.
I don’t know if ‘immunity’ is needed.
More importantly, at the start of Midnight, the Sunwell IS providing support. It is what is holding back the voidstorm. It is strengthening the defenders. And Paladins and Priests will be aiding it. So, the Sunwell can help without a need for a single character to be ‘super powered’ to win the day.
Personally, I hope to see Liandrin taking a leading role in the Sunwell’s defense. I want to see her character grow, not just get a deus ex machina power up for the sake of a power fantasy.