I will hazard a guess. The traitor is:

One of the knights

In the first phase diagram, he is on:

F5
We are told that in each diagram it is Boston King's turn. We are also told that the king sent some cavalry to be sacrificed, so we know how highly the king thinks about the knights. In this diagram the knight on F5 has the perfect opportunity to sacrifice himself to attack the Weshire Queen. It's a no-brainer, actually. However, since the queen survives until the end of the battle we know he didn't do it. Chickened out, as it were - or had some ulterior motives?

In the third diagram, he is on:

A6
This one is pretty easy, since we know the traitor doesn't change, and there is only one knight left. However, in this case there is also some evidence. We are told Boston King had a major card to play, but his queen got trapped and he was forced to agree to a truce. Now, if you want to play a major card but you can't because your queen is trapped, what do you do? Un-trap your queen!
The Boston queen is trapped by the bishop on F3, and the bishop is protected by a rook and the Weshire queen. To easily get out of the trap the knight on A6 needs to move to C5 and attack the Weshire queen. If the queen is no longer protecting the F3 bishop, then the B7 bishop takes him, then the rook takes, then Boston queen takes rook and Boston is up material. If the Weshire queen moves to E2 or E3, then the rook on A2 can take the pawn on D2. The Weshire queen takes rook, and is not protecting the bishop anymore. Bishop takes, rook takes, queen takes. Now Boston is still up a pawn in this exchange and the queen is no longer trapped, opening the battlefield up for this major card. So the knight really dropped a ball on this one - or did he?

In the second diagram, he is on:

D8
This one was a bit tricky, as there is no glaring evidence against him. But if you think about it, why did the knight move back behind the pawn line? He is not attacking any important squares there, and he is not defending anything. B7 is very well defended, C6 also has 2 defenders already, so does E6. F7 is not being attacked, and even if it was, there are better places to defend it from. There is no reason for that knight to have gone back to the 8th rank. To top it off, it would have required him at least 3 moves to get there - what a waste of time! Also he would have had to have gone through at least one turn where he could have attacked a Weshire piece, and where Weshire could have attacked him, but he got to where he was going unscathed. Collusion!!

Having seen this, the Boston King

Was understandably livid. I don't envy that knight!

UPDATE: I got it, the king thinks the traitor is:

I got it, the king thinks the traitor is:

UPDATE: I got it, the king thinks the traitor is:

The light squared bishop.
The one remaining knight is the king's son, so he is beyond reproach, even though there's a ton of evidence against him. Looking at the other candidates we don't have much to work with. Half the pawns didn't even move, the other half are battling as they should. The dark squared bishop hasn't moved all game, if he was a traitor then he is an ineffective one. That leaves us with only the queen, two rooks and the light squared bishop. In the first diagram the rooks are blocked so they can't be doing anything suspicious. The bishop is all the way in Weshire territory though, talking to their king - highly suspicious!
In the second diagram it is explained that Weshire "raided" horses from Boston on the right flank. Now how did that happen? Wait, who do we see on the right flank? The bishop! He's back, why? He is the one who helped Weshire get those extra horses.

But wait, there's more!

I'm sure the king thought up some excuse regarding the third diagram. But while I was trying to do the same, I found something very interesting: the king is an unreliable narrator! Regarding the first diagram the king says "It was very bloody and emotional battle". But if you look at the board, only one Weshire pawn is missing. Does that sound like a bloody and emotional battle? For the second diagram the king mentions that Weshire priest moved in a "never before permitted" manner, however, the one remaining Weshire bishop hasn't moved all game! For the third diagram, the king says the queen "moved to a bad spot" to talk to the bishop, but the queen hasn't moved from her place in the second diagram! So the king has no idea at all of what's going on.

Having realized this, the conclusion is that:

The king is a paranoid and unreliable witness. He thinks there is a traitor, where there is none. He ordered the arrest of the light squared bishop, but all the evidence is circumstantial. He's going mad in his old age...