There's only one realistic "fix" for the Mentor system as it stands right now, and that's man-hours.

To ensure that everyone only ever has perfect experiances with unfailing polite and informative Mentors, SE would have to have a dedicated GM team whose sole purpose was to monitor Mentors and deal with each and every accusation of abuse of the system. They'd have to personally check each and every sprout complaint, reading tickets and checking chat logs by hand, because there's no computer program on Earth sophisticated enough to determine tone and intent in human speech patterns. It would be such a resource sink that SE might as well just hire and pay people to be Mentors.

Because no matter what kind of requirements or restrictions they put on the system, no matter what hoops players have to go through to become Mentors, people are only human. There would still cases where an otherwise 'perfect' Mentor had a particularly lousy day, had enough of some pants-on-head stupid sprout and snaps.

And then there would still be these exact same "Mentors r tha DEVIL!!1!" threads. ¯\_(ツ)_/¯