Glad to hear she's still fighting. What a strong girl she must be.
Jane, you are in no way being cruel. From my limited experience I've learnt that you can read when an animal still wants to fight and when they've had enough. I'm certain you know how to read Elsa, better even than the vets that don't know her, and you know what she'd want to do. Rabbits are so delicate yet they can have so much fight. I was virtually told Jazzie wasn't going to make it back in Feb, even the exotic specialist consulting said she 'shouldn't be alive' but she got through it all against all odds. She's so timid and scared, I would never have thought that she had such fight in her, especially how ill she looked that night. Yet now she's probably the happiest rabbit I've known, she seems to appreciate life more (as crazy as it sounds) and I genuinely believe she is glad to be alive.
You know your rabbit better than anyone else and you know what's right for her. I firmly believe you should always give them every chance to make a recovery unless there is absolutely no hope or their suffering is so severe and you know that they can't go through anymore.
You are not being cruel to Elsa, you are giving her a chance to keep fighting and the fact that she's still holding on says to me that that is what she wants.
Please don't let anyone else tell you otherwise. You know her best.