Hi,
Are you using a valid MySteinberg account?
Can you tell us the exact message of the dialogue?
If your MySteinberg account is not working (at the moment), yes, please use the key-option.
Hm, good question. There are so many files. 
You should not replace the “Vst*.xml” and the “Defaults.xml” files.
No, VST Connect will logout when you are removing the plug or closing Cubase. You don’t need to care about it.
Have a nice day,
Michael.