Well then, when I make a plan it'll include studying these buggers, because these are a possible problem just like blink spiders.
When does the vote open, by the by?
convincing them to become a successor chapter may be possible, though only if you can confirm that the main chapter lives
On this matter, I would simply tell them that our intel captured from the Dark Eldar indicates that Ultramar was noted as being one of the holdouts noted as surviving and likely to continue to do so, and let them make the decision. We don't need to force them to rebrand if they don't want to.