I've told you what was at stake. I've told you what it was that the player was putting on the line. I'm not sure how you think that you - who were not present - know better than me - who was present - what was at stake.
I'm also not sure why you continue to frame the stakes in terms of material reward. I've already explained that that was not the essence of the situation.
Because that's what you've indicated the results of a success would be -- the finding of a non-magical mace. What, praytell, would be the big reward that accompanied the finding of a mace?
Look, I see that the framing was of something big -- the return should have something interesting involved to justify the framing. But the check was to find a mace. A nonmagical mace. That's not high stakes, that's pretty darn low stakes. You didn't offer anything on the success side to warrant the high cost failure. If you want a high cost failure on one side, you should balance it with a big reward on the other -- not a mace.
I also don't understand your use of the word "arbitrarily". This wasn't arbitrary. It was reasoned, and conformed with an "intent and task" approach to adjudication of failure: the PC's task succeeded, in so far as he found items that the brothers left behind in the ruins when they fled; but his intent failed, in so far as what he found was not what he had hoped to find.
The arbitrary bit was that you could have decided to put anything else there -- a monster lurking, a clue to some other puzzle, anything, but instead choice, arbitrarily, to provide evidence that completely reframed the player's backstory (you authored part of the player's story for him) and that invalidated one of the player's main goals in play. That's looking pretty arbitrary from here. Did you skip telling us the part where the player asked for such a possible outcome? Had I been the player, I would have been upset that the looking for a mace had that result, especially if it was spur of the moment.
On what basis do you assert that the risk was hidden? I've told you what was at stake. The player knew that this was what was at stake. His PC had brought the group back to the ruined tower, the first time the PC had returned there since fleeing the attacking orcs 14 years ago. This was a big deal at the table.
The player knew he was risking one of their goals and a major change to their backstory when they went looking for a mace? Okay. I suppose if you told them you'd negate one of their goals if they failed the check to look for a mace and they went with it, I don't have a problem with that. However, you haven't said this up front, yet, that you established the stakes before the check was made.
From memory, the player attempted - and failed - an aura reading check to confirm his confident belief that the arrows were not made by his brother.
It's very, very difficult to argue a point based on your presentation when you don't present the whole story.
But, again, on this failure, did you author that the brother was evil or did the player? Finding cursed arrows, even knowing they were made by the brother, would not prove guilt to me. There are a ton of other explanation that fit, especially in a game where facts are ephemeral until a failed search for a mace occurs and then they pop out and influence past events.