You are correct - there’s no activation path on macOS and Messages / FaceTime only absorb a phone number when it’s not assigned to an iPhone used to sign in to your iCloud account.
You added a multi factor sms and not a reachable link via FaceTime (good effort here BTW it was worth a shot).
To do what you seek, get that number on a SIM and into an iPhone ...
Can I be reached on FaceTime on Mac using my non-iPhone phone number?
No. You need an iPhone to do that.
I've added my phone number to my Apple ID
What you have done is associated your mobile number with your Apple ID. The mobile number can be used to receive verification code when logging in with your Apple ID (if you do not have an active trusted ...
Apple covers this expertly.
Once you have a backup, you can restore the documents and not the apps or restore the apps and delete only the ones with an embedded AppleID that’s wrong.
With this procedure you don’t need any access to any AppleID or any previous passwords.
You should go to https://developer.apple.com and look for the Account link at top of the page. You should be able to create a free developers account. If you are developing for macOS or iOS you'll need this account sooner or later anyway.
As of Aug, 2019, the situation is still unresolved.
After a conversation with Apple Support it transpired that the ability to change an AppleID that ends with @icloud.com to a mail alias that end in @icloud.com is not implemented.
However, it is possible to do this if your mail alias end in @me.com or @mac.com, which is what the support article mentioned ...
You can’t split an AppleId so either one party keeps it or the other keeps it. Then you remove the account from all devices not belonging to the new owner.
When you delete an account, you can choose to retain the data locally or erase it. Without a backup, this choice is permanent.
Since app purchases cannot be transferred
Data like Safari history(other ...