The threshold for safeguarding issues is astronomically high, just because the court have said this does not mean that the mother may not have genuine concerns about the children whilst in your care. Something like going to bed at a different time, issues around diets oand the other things you mentioned can be hugely disruptive, court would not describe this as a safeguarding concern yet it might have a significant impact on children.
Your claim that the mother is fabricating illnesses is extremely serious and I suspect if you took a less hostile view of her you may not frame it like this. I also strongly suspect that the medical professionals see has interacted with during these times have not accused her of this, because I think you would’ve shared that in the OP.
It seems you are also embroiled in the conflict. If the children have picked up on this, they will resent you and your DP for it, as much as they will resent their mother for any snide remarks she may make. I’d say this from experience, both as a parent and as a child.
it’s incredibly hard. The courts are not going to sort this out, every time you go the conflict will get higher and everybody, not just the children, will be hurt by it.
my advice is to lean into reassurance, kindness, and trying to take heat out of this situation as much as you can. It’s the opposite of how you feel I understand, but it’s such a powerful thing to do. being polite, being kind, saying nice things to the kids about their mother, and trying to understand her anxiety, rather than dismissing and trying to shut her out.
You are tied to together while raising these kids, this is just pragmatic. Doesn’t mean you have to be doormats but if you can approach interactions with warmth and understanding you may find things get easier