Let's start with your second question - no, you can only crated Integration Keys (IKs) in demo/developer account.
Now, you may be confused a bit about the relationship between IKs and accounts.
Once an IK is approved in prod (go-live process) then it can be used with any account in prod. It can be used not just with the account that created it but with any other account, completely unrelated to the organization.
So, if that's your need, you're in luck, you don't need to do anything different.
Create an IK in demo. Get the app working in demo (Developer account) and test it there.
Once your IK is approved (passed go-live) it can be used in production with many different accounts.