Board Thread:What If?/@comment-173.245.80.12-20140903054558/@comment-37017073-20190525231741

Hakka84 wrote: So. Yeah. It's frustrating that the same guy who accused Sir Valiant of using magic by showing up in front of the king with the severed head of a big snake, is believed dumb or close-minded enough that The Powers That Be In Canon (namely, Gaius and Merlin) decide that he's not to be shown the necklace that killed his father...

I think the difference is that in Valiant, Arthur's willingness to believe Merlin was primarily based on honor. He was clearly skeptical of what Merlin was claiming (even with the snake as proof) but chose to believe him and take the matter before his father anyway because Merlin swore to him that he was telling the truth.

Merlin also had evidence to support his story besides the snake (namely Ewan's testimony, without which their case fell apart), so Arthur wasn't just blindly taking his word for it, either. The same could not be said if the necklace had suddenly convinced him that Morgana was responsible in The Wicked Day. Nothing about the necklace singles out Morgana as the guilty party and there's nothing to indicate that she's been anywhere near the castle (much less that she has a spy in their midst), so for Arthur to disregard the obvious suspect right under his nose (Dragoon) and shift all of his focus onto her just because Merlin and Gaius say so wouldn't make much sense.

...nor the bracelet that could explain why his almost-wife suddenly and illogically betrayed him the night before the wedding ceremony.

Nobody knew about the bracelet, though. No one was around when Shade Lancelot gave it to Gwen, nobody noticed her wearing it, and Gwen threw it away while she was in the dungeons. No one was keeping that bit of information from Arthur; they genuinely didn't know it existed.