I got this game so that me and by brother could share it and have separate accounts. When he installed the game we entered the registration key on his account.
I installed the game on my computer and entered the key on my account. When I tried to log into the game it said "account error". Then I realized that the registration key can't be shared. However, I get no message saying that the key is already in use or anything like that when I enter it with my account, and it says that I'm subscribing to the free 30 days, so I don't know what the problem is.
Yes, of course you can, just publish your key for the rest of the world to use, so we can all get a free copy.
NO, when you purchase a retail copy you pay for the license to use the software, the sub is required to ensure continued access to the service. A separate license is required for each account that is setup to access that service, even if it is the same account holder opening multiple accounts.