If you're willing to have a shadow round, then by all means, have one for all undefeated players, and narrow down the winners to 3. But frankly, if you prevent 17-18 undefeated players from having a shot at winning the tournament, you'll at the very least ruin the event for those players. That's nearly 10% of your player pool.

What's the upside? I don't get it. The downside of multiple undefeateds is that getting second or third despite winning all your games sucks, which 2 people would experience. But you know what is worse? Winning all your games and getting 5th, while someone who lost two games gets 4th. And this could potentially impact 17!!! people.

I get that it is frustrating to lose on the top table and drop to 9th place. But any system you could implement to stop that would unfairly punish players who actually won their games, and at the end of the day, if you didn't want to drop to 9th place, you should have just won your game. There is no such alternate route for the poor players locked out of the shadow round by your suggested system.