It also defeats the point of the trap itself - what proof would they have, that his intention was kidnapping, when they didn't even give him the chance to try? If taking the MC at his word was enough, they should have just grabbed his ass immediatly without any subterfuge. If the guy wasn't suffering from author-induced artificial stupidity, he would never have grabbed a hostage, just stood his ground and invoke any number of excuses why he needed to see the girl.
The trap itself makes sense even if it didn't need to be Eta. The Village Leader still wanted (and possibly required to enact punishment) proof of wrong doing by his right-hand man.
Just by going INTO the secluded hut the traitor was already guilty, like what would be his excuse?
"No no, you misunderstand! I didn't want to kidnap your young daughter! I merely meant to rape her. Surely the GODkin wouldn't mind the harmless prank of a stranger raping his wife on their wedding night."
Like maybe if the traitor stayed outside of the hut continuing to try baiting "Eta" out of the hut by feigning an emergency or maybe saying one of her friends in the village wanted to meet with her, he'd have half an excuse... Until when they ask - "Hey idiot, how does standing outside of the hut spewing lies to lure our daughter to an even more remote location clear our suspicions of you?"
Again, I agree that it was indeed pointless to use Miya instead of Eta (Really the Father could have been the person waiting in the Hut with trusted guards), unless they think/know that:
A) The traitor was watching from a distance and they needed a smallish vaguely female silhouette to enter the building to fool him.
B) The traitor may have had a plan to enter the hut in advance or to have an accomplice enter the building (either before or after "Eta").
If you think about it from all angles - The Husband, the father, and the traitor had to all be visible during the ceremony - it would rise suspicions if the traitor was aware that they were not present for the festivities AND it was important for the traitor to see what they were up to until he made his attempt to kidnap Eta.
If the Traitor didn't care to keep an eye on the Groom (MC) and the father - He could have just feigned illness and tried to snatch Eta at night without confirming that everyone was were they should be - unless he had an accomplice to be his eyes (which he did not have according to the story).