I realize you can add a domain for hosting, no matter its nameservers or current hosting, but that doesn’t establish routing to it via dns.
There is some situation where I have tried to add a subdomain to hosting without the domain being there, and I have gotten a ‘you are not authorized to add this subdomain’ error message or something similar. I had interpreted this as the dns issue I laid out in point no. 3. So, is there a very similar situation to this where that error came up due to the way I thought of it, or could it only be coming up for only the reason you gave, i.e., just because the domain is not added for hosting first?