Officially you can, but it doesn't respect it. It won't share the posts directly, but it'll still suggest you to your friends based on it.
Not that I could find. I remember trying to use an alternative email even, but it was my back up email on Facebook and it linked them automatically.
If this is the case for you, maybe it only applies to newer Instagram accounts since I had mine long before the acquisition and my posts go to FB only by choice (and I almost never choose that anymore).