My personal opinion is that you shouldn't worry about a game being cloned in the jam. It is extremely incommon and, even if it happens, gamedevs know A LOT of games and they will call it out. I usually use my game jam games for commercial purposes and this rule is a problem to me, but if it isn't a problem for other people, it's ok and I can think about a game that won't have any commercial value.
About the "bad manners" of people voting down other games, I don't know... I have participated in jams with prizes and the best games were still at the top. With a lot of voters, the best tend to stay on top. But yeah... I guess having charisma is part of the event as well.
The rules are not clear to me as well: Who decides the biggest winners and how is it decided? The easiest ways are through the itch.io voting system or through a jury. But the criteria and rules have to be known.