There's no denying that (or intent to try to), but I much prefer the way they did it. A whole lot of people wouldn't have identified the song right away (I didn't, I just assumed from Cat's reaction it could only be the Rains of Castamere), but that didn't matter as long as it was very clear Cat had figured out something was up, and the acting was really great and conveyed all that and more. It was also perfectly fine there was a little confusion about what tipped off Cat about the music in that scene. My comment was more in the order that I liked the fact they didn't guide the viewer by the hand and rather let them puzzled out what about that melody made Cat react as she did (beside the fact it was also badly played). It would have been totally different if everyone there was supposed to have noticed it too and only the TV audience was clueless.

Using the lyrics in the scene would have been a big cheat - the guests were never supposed to figure it out or it wouldn't have been used as a signal. A lot of people there would know of the song (and its lyrics), but few in the north would have heard it sung or had heard its melody played - but Cat is from the Riverlands, close to the Lannisters. Had the lyrics "rains of Castamere" been heard, a whole lot more guests would have been alerted (if not at the trap then at the slight/insult) Production wise so much was happening in that sequence they needed to carefully pace and choreography it all, and let each "beat" of the drama play out and pay off. The pandemonium couldn't start too fast, before Talisa got killed and Robb and Cat were positioned for the next scene. Having Cat mutter the title to herself would have been another cheat, and have her spell it out aloud to the room would have created total pandemonium (and remove Roose's reason for not attacking her right away).

Identifying the song right away by puzzling it out was in the end just a bonus for viewers and almost a nod to book fans, they didn't make understanding of the scene rests on that and, as I said, at that point a little bit of confusion about Cat's reaction was a positive thing. It wasn't yet time to gasp, that came when Talisa was stabbed.

In the season finale they can bring back the song in KL's.

Yes and no. I think it was definitely a writer rather than network decision. A character like this provided far more opportunities for Robb private battlefield scenes and for a more TV-friendly romance. She provided a counter to Cat's influence after Cat's mistakes. She provided here and there a little insight into one of the slaver cultures as Daenerys's own story unfolded. In the book it's all tied to Ned, Robb doing a Ned-like honor call on his own which was also a terrible political decision, then it centered on Robb and Cat. Beside providing material for Cat to reassert her influence, it had little potential for TV unless they showed these events. The Talisa option let them do all this without interrupting the campaign, and with having Cat present, which allowed more of a build up of mistakes by Robb instead of this being presented as a done deal as in the books.

The clues she was up to something were really to get the book fans talking and guessing about her until the end, which worked. But storywise making her Tywin's spy simply would have propelled her to "major villain" status and thus stolen Roose's thunder at a bad time (it's Roose who keeps going and needed more spotlight). Robb's wife made too huge a traitor.

For the rest I suspect they intend to use Lady Stoneheart more often than in the books (show behind the scenes events à la Theon not to lose the character) and they neither wanted to submit the actress to four hours of make-up with prosthetics every day of filming (the throat they can choose to show or not show when they wish), nor did they wanted to foot that bill till the end of the series. She can hide her face when she appears and shouldn't be recognized by other characters.