Also, I've read the books twice and I didn't even think that a letter would have to be run by a Maester. To suggest that this is a glaring error that would have been picked up by a TV viewer seems a little silly to me. Maybe Lorch's retinue carry their own ravens and he dealt with the letter through his own people? Hardly more of a change than any other we've seen on the show. You could just as easily say that Theon's thought of writing to Robb was silly because it would have to have gone through someone else on Pyke.
Ultimately, I think all of the Harrenhal plots have been worth the changes simply to see the resulting Tywin/Arya scenes. I was sceptical at first of the decision to switch Tywin and Roose but I think it was the right choice. This episode was one of the best so far in my opinion.
To be fair, why would a Lannister host have a raven trained to home to some minor house in the North? "Send this to letter to House Mumble forthwith." "Sorry ser, but we have no ravens for that house." Is how I see that conversation going in an internally consistent sub-sub-sub-sub plot within the larger story.
Now in that context I do see Amory Lorch handing over a letter saying "Send this off immediately" and walking away, then the maester, or whomever, looking at the address and going "What the...? I better run this by his Lordship to make sure this is where he actually wants the letter to go. At which time his Lordship facepalms and decides to give ser Amory a good old fashioned dressing down."
There are credible ways for Amory to mis-direct an important set of orders or whatever and for this to come to the attention of Tywin. And Tywin being Tywin he has little time for incompetence, and given his later scene talking about Jaime's dyslexia Tywin would be inclined to treat another dislexic with a degree of contempt.
So if a raven actually got sent to the wrong place, that's not credible. But a semi-literate, dyslexic Amory Lorch putting the wrong address on a letter and this being reported through to Tywin is kinda credible. Why Amory had to be on the out with Tywin is a whole other question regarding time wasting of precious minutes on trivialities, as it doesn't really seem necessary given he would react to Arya having that letter the same way if he was in Tywin's good books.
What I could have accepted as a valid reason for a scene where Amory gets a telling off from Tywin is Amory subsequently being in a particularly shitty mood and abusing Arya with a smack around the head for her spilling a bit of water on his boots or some other minor inconvenience, at which point Arya hotheadedly wastes another assassination in an act of petty vengeance.