Having Brooker as King of the Zombies might be slightly too ridiculous. BECAUSE OBVIOUSLY WRITING ABOUT BROOKER AND MITCHELL IN A ZOMBIE APOCALYPSE IN THE FIRST PLACE IS PERFECTLY SANE. Having a leader and following decrees does seem a level of rationality slightly too far beyond the moaning, grasping reach of traditional zombies, though.

Immunity (perhaps for only one of them?) could work! It doesn't strip out suspense by removing the threat entirely, as, as you say, it won't prevent the zombies from tearing them apart if they have the chance, and it allows for the drama of 'OH NO YOU HAVE BEEN BITTEN but I cannot bring myself to kill you!' without too unpleasant an ending.