You need to understand the difference between an incorporator and a board member. An incorporator is an agent for the single task of incorporating the company. That's it. It is usually prudent attorney practice to have a signed resignation, but this sounds like you are doing this yourself, so you did not do this or even know this should be done. As a general matter, even if the incorporator did not resign, their job and authority is done on incorporation.
An incorporator does not automatically become a board member. What should have happened is that you should have had minutes electing the board, the board then has organizational minutes that approve the bylaws, elects officers, and the like. It also sounds like you didn't do this either.
So if you want to avoid problems, hire an attorney to get the documentation done correctly or learn the law. Making a mistake and haphazardly hiring and terminating board members is a sure way to create loads of problems. Good luck.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.