I answered that question when I brought it up, this is a story, it's like asking why isn't it more common for heroes to get powers easily, it cheapens the whole thing.
No it's not irrelevant, it happens in the story for a reason, to make it more interesting. Maybe the author can still with the exposure reasoning or maybe Xanthe will later prove to be an unreliable narrator.
Yes down to luck, or effort, like how almost every superhero is made.
The serum cheapens the process by making it a pill to pop. That is usually reserved for mass produced enemy fodder, not superheroes.