As someone who hasn't really been following the current X-books, apart from the odd exception, I'm pretty indifferent to this, albeit mildly curious. I read about this theory years ago, so interesting to see it brought to fruition.
For me, about the only non-negotiable aspect of Nightcrawler's background is Mystique being his mother (or father, as in this case, I suppose
). Frankly, I was disappointed that the movies never really explored their relationship, despite
both the original trilogy and the First Class films featuring Kurt and Raven in scenes together. Azazel was promoted as being Nightcrawler's father in the lead-up to X-men First Class and that movie ends with Mystique and Azazel both ending up as founding members of the Brotherhood, so it really felt like they were setting up Kurt's birth...only for them to have Raven meet Kurt in X-men Apocalypse with
zero acknowledgement of their relationship (in contrast to the Magneto-Quicksilver relationship which is very much acknowleged).
As for the people saying that this would be too controversial because of the trans themes - honestly, I don't think you can compare what Mystique does to any kind of real-life transition process. Just treat it as a weird sci-fi thing.