1) Yes. This is not a spoiler issue. You have to warn people about upcoming Elsanna -- even if you put the warning at the top of the chapter when the theme changes.
There is a trigger warning rule about things like rape and violent sexuality which dictates you really MUST indicate your intentions about introducing themes which may make the victim of abuse suddenly very uncomfortable. Technically speaking most people who are objecting to you suddenly going Elsanna are probably angry over the sexuality issue. But the reality is that you should have been open about your intent to introduce Elsanna as a warning to people who have been the victims of incest or sexual abuse by a caregiver/authority figure/person of trust.
When it comes to trigger warnings in fiction, the reality is that if you have a lot of female readers, rape is a shockingly common experience and so you probably the need to actually use the appropriate terminology when you go there. And that is appropriate.
Because incest is not so common it feels silly and overly politically correct and SJW to say, 'trigger warning/incest' and it really would be silly to use those words. You don't have to do that. But you do have to give readers an accurate warning. You can't just spring it on people as if Elsanna is not similar in any way to a very abusive and damaging relationship.
You should even make sure no one expects A-KA style sibling Elsanna if you are going to swap to M/icest. So when things change from the Kristanna & Elsanna (sibling supportiveness) in the first chapters to icest you may need a chapter heading warning.
As for the Kristanna shippers who may be angered when you go 'eventual Elsanna' many people who are open about gender preference sexuality may have issues with fluidity. If you don't want the anti-gay or anti bi people flipping out on you, you need to warn them that Anna isn't 100% straight or gay. So you need to include Kristanna and eventual Elsanna in your description to warn off the haters.
2) You may be able to pass off Hans/EvilElsa/Duke as an OC or just not include them in the summary. Onion/Evil Elsa especially can be introduced without warning. Couples are generally included in the summary but not all the characters. In fact the villain is seldom mentioned.
You will miss the Hans fans if you leave him out of the summary but they may not like how you've described him anyway.
3) I dunno. I think you might attract more readers with an honest description that gives away your ending since so many dark fics have been so bad recently. But I've never been spoiled by spoilers -- knowing the ending has never stopped me from enjoying a good story.