Your beta code is permanently attached to your account. If you want to attach your Microsoft account to your google account, you need to have another sign in option connected to the account that you are trying to leave behind.
So have you tried doing that? Because each account requires one sign-in option.
This is from a third account linked with Facebook that I cannot link with any of the others.
You can't unlink an account you are signed in too and you can't unlink an account you aren't signed in to.
You should be completely able to link or unlink another account (just to check, I am assuming the accounts we are talking about are sign in options such as google) as long as you aren't signed in through it. So if you wanted to link or unlink your Microsoft account, you can't be signed in through your Microsoft account.
So what messages are you getting when you try to unlink an account?
I have an Android phone and another computer on the same Network as well as my Xbox on. I had to disconnect Google Chrome on each PC and shutdown both my phone and Xbox. Finally I could link them. Thanks for the help MiloOmega!