Sanity, the OP was due to go around and cook dinner for the DC anyway, so if the DD was saying she missed him, the mum could simply have said Daddy is coming round later.
Anyway, sorry, it was me that started that tangent. I think it is good that you will make sure and have contact separately.
I also think it sounds like she has been planning this for a while. But I think the point about her saying gifts will still be joint shows that she doesn't grasp the reality: the OP has a say in that too, if he wants his gifts to be from him, and given by him, labelled by him etc., they can be. That needs to be decided by them both.
I also think that the reasons are kind of immaterial; if she wants to end the marriage, she wants to end the marriage. If that is clear, and it sounds like she made that clear, then it is unfair of her to start muddying the waters. If the OP wants to be sure, he can ask if she really does wish to end it, but I wouldn't go snooping, or involving strangers, or pressing her on the matter. You don't want to look like you are harrassing people. Whatever she does, has done, has not done, keep yourself right. Whatever you feel inside, behave with dignity.
As I said, good that you are taking legal advice. It is worth the money. A good lawyer will inform you of your rights, your DC rights and seek to protect these, whilst also trying to keep it out of the courts. You don't need to tell your wife you have taken advice; you can see how things develop before instructing your lawyer to do anything.
All of which is a world you did not anticipate being in. I am sorry that you are. Look after yourself and take things one step at a time. It is not a good place to be, but you will get through one way or the other.