SPOILER ALERT. This section contains solutions.
“It is unclear whether the author intended for the unmarked doors to be an alternate solution. Since we have no direct statements from Christopher Manson on the subject, we can only speculate on his intentions based upon indirect evidence. Such speculation is inherently risky.”
This above quote (from John Bailey’s site) has now been confirmed. Unnumbered doors are not accessible and would ruin the integrity of the Maze.