StepE, I think your anxieties are normal. It's natural to be nervous about a new situation, especially when you're new on the scene as it is.
Please remember, though, that dh is getting a gc - NOT a new child of his own. It has nothing to do with whether you can give him a child or not. He probably wouldn't want to start all over again raising kids, anyhow. That's not what this is about or shouldn't be, as he won't be raising his gc.
I know you're worried about dh being away "on a regular basis" to visit gc. It may not be that often though, as the parents may be too busy to entertain or want to have other visitors, too. And maybe you'll be invited, also? Would you go? Regardless, I'm sure, after a while, you'll both fall into a routine that you're comfortable with.
Imo, you just need to go with the flow. Don't push to have more of a role in baby's life than the parents would like, but don't let them or dh push you to have more of a role than you're comfortable with. Also, I agree about the name. Unless they come up with something you really detest, let the parents decide what you'll be called, even if it's your first name. In fact, what they call you may give you a clue as to how they see your role.
I've got another 'keen'... Ouch!