I'm so happy that you've found a baby to give your heart to. He will help you in so many ways to heal.
I understand about the guilt feelings, I had the same when we got Gracie Ruth so soon after Annie B passed on. But I know how much Annie loved me - and I her. She never liked seeing me sad, she'd always try to cheer me up, so I know she wouldn't want me to be sad because she had to leave. I tried to remember that if I was the one who had to leave, wouldn't I want Annie to be happy with a new Momma. I'm sure Millie would feel the same way. She will be watching over the two of you and will be so happy to see you being the good Mom that you are to another baby.
As for a name, there are sooo many possibilities for boys. Think of a favorite place, food, famous person. When we got Yogi, there were so many possibilities. I love ice cream and thought Spumoni would be cute. My grandfather played a violin, so thought Stradivarius might work. My family is from Ireland, so Dublin was a possibility. But really when you already have a Boo (Boo-Boo), Yogi was the only true option. For those that don't remember or know of the Yogi Bear cartoon, his sidekick was named Boo-Boo.
So very, very happy for you. Please post pictures when you can. I know it will be a long 2 weeks.