I don't know if you wouldn't succeed in not sending her at all. I just wanted you to think through the facts and your reasoning.
I think you felt much stronger about it when DD said she didn't want to go. However, she then FaceTimed him and that changed and now she says she is missing him. She is obviously confused, the poor thing.
His slippery eel act, changing his mind and gaslighting you, has in turn confused things in your own mind. And I don't blame you.
Re the risk being to him, not DD, I've thought more about this.
I think he is at greater risk of catching it, not just having more serious symptoms. This is because of the steroids. I had to take steroids recently and the GP warned me they would make me super vulnerable for a while. Someone else on here may be able to add some wisdom here - was I more vulnerable to catching it because the steroids lowered my immunity (that was my understanding)?
I think you have a good compromise in suggesting splitting the time into smaller chunks. What exactly is your reasoning here? Is it just the being cooped up? Agreeing that relies on him being sensible. Which he isn't. But if you apply for it you have to have reasons to show it's better for DD.
YY to agreeing clearly what happens if he or his DP gets it. Moving her back to you would actually be against govt guidance as you'd be moving someone exposed to the virus into a healthy household. If he won't agree what happens I'm not sure a court would order this. Would it be the end of the world (for DD not you) if she did have to remain with the DP for a quarantine period?
What some people are doing to get around this issue and ensure that contact continues, even where one household shows symptoms, is to agree to treat the 2 households as one unit for lockdown purposes.
So you have household A and B. Someone in A shows symptoms. Household A must self isolate for 14 days. So does household B, because it is the same as A even though nobody there is showing symptoms. Child continues to move between A and B throughout. I didn't suggest this yesterday because in your case you also have household C where the DF of your other DCs lives. I don't know if there is also a household D (does the other DF have a new partner, does she have DCs?). So this suggestion may not work for you if he wants to try to complicate it by bringing in these other households (and to be fair it does complicate it).
He may also not be willing to treat himself as a single unit with you if there is an argument that this exposes him to greater risk (from household C - following the model, if someone in C shows symptoms they would return to A exposing everyone there, and when DD next goes to him at B she may be contagious)). On the face of it his objection on that basis would be reasonable.
On the face of it, it is entirely reasonable for you to try to agree with him what happens to DD if he gets symptoms. But there are no guarantees that he won't later try to keep her, whatever he agrees to now. So whatever you agree should be recorded in a court order and you should get the solicitor to help draft it.
Re mentioning his prior abuse, I'm in 2 minds. Has it ever been mentioned before in previous proceedings? There is a risk that it could backfire by making you look paranoid and hostile. It just depends on which judge you get and how you come across. In this instance, is it actually relevant? I suppose it is to the extent that you are saying he isn't thinking what's in DD's best interests he is just focussed on getting his own way.