That’s well written. I think that requiered 2+ code review could also help because with time more people will gain knowledge of the dark parts of the codebase, just by reviewing the PR of “Martin” when he work on them.
That entirely depends on how well code reviews are managed. I’ve worked with a “Martin” in the past and we did manage to move to a system where 2+ reviewers were required but it simply got to the point where no one would “rock the boat” because he’d simply brush off every comment made, or call you up to have a long rambling conversation as to why he made the decision he did and how you’re wrong and he’s right, and given his position in the company you couldn’t complain to anyone else about him because he was more valuable to them than you were.
We tried to put more and more blockers in front of him to attempt to encourage him to play nicer, but these were only temporary solutions to the bigger problem of “Martin” himself.
If the code base is arcane enough, code reviews won’t matter. You just won’t understand at all what is happening there. And the “Martin” will probably pressure you to accept anyway by telling the bosses “I can’t work, they won’t accept my code reviews”.