My kids are 5.5 years apart.
My older one, DD, was very excited to get a new sibling, but she definitely experienced a lot of shock when we brought him home.
We spend a lot of time teaching her that we need to celebrate others in our life, and it's not all about me/us. When we go to parties, it's not about the treat bag, but about celebrating our friend and trying to pick out a gift they will like. We work on teaching her to honor us (parents), her grandparents, and her teachers.
Most of us are naturally selfish individuals, and your DS (or any child, for that matter) is no exception. He's being selfish--plain and simple.
Call his attention to it. Let him know that life is different now, but it can also be better. When my DD gets frustrated with my DS, I remind her that God blessed her with another person in the family to love, and that they should always be there for each other. We remind her that when she was young and the only one, she got all the attention. And now, it's time to rise up and grow into the person God wants her to be, and that does not include selfishness. It's time to celebrate her younger brother, just as she wants us to celebrate her.
It sounds like your dh is very hands-off with the kids. I hate that for you. It sounds like you realize that you cannot change a grown man, and you're looking for ways to address issues with your children yourself. Kudos to you for that.
I wouldn't put dd in the daycare the entire time you're there. That's not fair to her, and it sends the wrong message to him. DS sounds spoiled, and I wouldn't cave to him any further. If he wants to belly-ache, remind him that Disney trips are a special privilege, and no one is entitled to them. If you think this is going to be miserable (and from what you've described, I suspect it will be), I would cancel the trip and reschedule for a time either Dh or a friend or relative can go with you.
Also, I think you're wise to seek counseling, even if you go alone. You have a lot on your plate, and a counselor can help you better determine what can be delegated, what can be let go entirely, and what's yours to tackle head-on.
I wish the best for you and your family. You sound like you have a heart of gold.