What should one do who led people in the Maghrib prayer, performing it as two rak’ahs, thinking it was permissible to shorten it, and then re-prayed alone without informing the congregants?

You erred in shortening the Maghrib prayer, as it is not to be shortened by scholarly consensus. You must inform those who prayed with you about the mistake so they can repeat their prayer. This might even be obligatory for you if you are quite certain that no one else will inform them.