If you added the domain to a Preview account, you need to fully remove it. That's only possible if you delete all users by creating an unlicensed admin user and using that to delete any users. Once it's removed try verifying it again in the P1. Agreed, none of this is made clear or intuitive by MSFT. The Preview isn't even in beta so they really should have blocked people adding domains until the service is a bit more stable.
Keep in mind in a few months your P1 account will be "converted" to the same set up as the current Preview.........so you'll be back where you started.
