The Horde itself makes no provision for this. The only thing the Warchief needs to be legitimate is to be appointed, then they're the object of the oath and presto, that's it. It's kind of what you get when succession is based around whether the previous guy in charge liked what he heard about someone he just met, or the candidate's dad was big buds with the present Warchief or he was dying and high off his ass and just selected whoever was closest.
That having been said, the Mak'gora being ignored argument still doesn't work. Setting aside that Sylvanas wouldn't really need to cheat to beat any of them though, refusing the challenge or cheating discredits her basis for power. Think of what happened in the movie with Gul'dan and Durotan. If they challenge Sylvanas and win, cheers, they can redirect the Horde to be whatever they want as they'll be the Warchief. If they lose because she cheated, they're a martyr that turns the Horde against her by proxy, so they still win. Same if she refuses to accept the challenge. The only scenario where they'd lose is if they lost fairly.