I focused before on Tarrlok's death possibly effecting Korra's character development. Now I am going to mostly focus on the plot aspect on why Korra should find out about Amon and Tarrlok's murder-suicide.
I would have abandoned this idea a long time ago like I did with many of my lame half assed theories, but I keep coming with more and better reasons for why Korra should find out. It has been 9 months sense the finally and in this time period I haven't come up with a single logical counter argument at all!
Not having Korra find out would be a plot hole, jumping the shark, and a cop out all at once
The Plot hole: It would make no sense why everyone would just forget about/take their chances on letting Amon, the most dangerous water bender in the world and the terrorist who took Republic City getting away. It would be very out of character for Korra to be very sympathetic to Tarrlok's child abuse story, want to help him escape, thank him for his help and then not notice or care that he disappeared… FROM HER ATTIC!!!!!
This would also be an "aborted arc" when a story arc disappears off the face of the storyline, and is never to be heard from again. Korra was upset with Amon getting away, oh but next season she suddenly doesn't care about that and never brought into question again?
Jumping the shark: There are still many people who don't like Tarrlok; it would be like they decided to not even bring up Tarrlok next season just to make these people happy. Also showing they ran out of ideas regarding Tarrlok a major character, someone who could blood bend whenever the hell he wanted, what's going to top that? Nothing, And why should he be topped? When this show involves the super natural there should be no excuses for why Korra can't find out, Aang in the book 1 finally of ATLA went to seek Koh for help who knew about Zhao's plans to kill the ocean and moon spirits when no one else did and Wan Shi Tong's name meaning "he who knows 10,000 things" it should not be any different with Amon and Tarrlok's murder-suicide.
The Cop out: Some people already felt it was a cop out to use Tarrlok to give us the revaluation of Amon's identity, a way to defeat him, and kill him off. If they then throw Tarrlok down the crapper by not making his absence important to the rest of the plot it really will be a cop out. I've done my research and according to tvtropes.org (where I learned about many of these other examples) this would be under the "stupid/senseless sacrifices" cop out or poor character development that the characters shouldn't give up their lives for nothing, and if Korra doesn't recognize Tarrlok's sacrifice that's what it will be. Tarrlok met his pre-mature death and next season Korra is using avatar state to win an air scooter race with little kids, THAT IS A DISGRACE!!!!
It's called a "shocking swerve" when they make a twist ending just to have one. I think it was really the only great emotional way for Amon and Tarrlok to leave the show, but having a murder-suicide just to have a murder-suicide is just cheap. Korra don't go having an identity crisis on us and start thinking you're south park.
The plot of LOK so far has been set up opposite to ATLA going along the Asian philosophy of opposites Tarrlok's death would continue this trend:
The two series have plots involving child abuse: in ATLA with Zuko it was the recovery story, this time in LOK with Amon and Tarrlok we got the horror story.
Aang losing his people from genocide and many people being angry at him for disappearing for 100 years, Korra losing a former enemy to murder-suicide and everyone thinks she is a hero for saving Republic City from Amon and don't know or even care what happened to Tarrlok. What Korra and Aang would share in their situations is that they both feel responsible for the tragedies.
So many people hated Tarrlok at the beginning but then many including myself felt really horrible for him at the end. Although prettying up Tarrlok's character at the last minute with the sob story to help out the heroes isn't going to cut it forever unless it causes more conflict in the future. Aang saved Zuko's life when they were still enemies and it ended up saving everyone. If Aang left Zuko to die he would have no fire bending teacher and no one to warn him about Ozai's plan and the world would have ended!! So is something bad going to happen because Korra wasn't able to save Tarrlok?
Book 2: spirits opens at festival celebrating the expansion of the southern water tribe and always In ATLA just when our heroes think it's time to celebrate/take it easy only more challenges and hard ships are waiting:
1) Book 2 earth of ATLA when arriving in the earth kingdom team avatar is praised for their victory in the North Pole they shortly afterward run into Azula
2) When Aang and his friends decide to take vacation Appa gets stolen later that same day.
3) At the end of book 2 earth things are starting to look brighter for team avatar but soon after Azula snuck into Ba Sing Se and tries to kill Aang making them loss the battle
4) In the ATLA series finally Aang thinks he can take it easy after all the hard work then Zuko drops a bomb on him by telling him Ozai's plan for Sozin's commit
5) In the promise trilogy they celebrate the war being over then only a year later they face problem trying to figure out what to do with the fire nation colonies.
6) After partying in the south the northern water tribe stirs up trouble with the south and on top of that will Korra find out the horrible truth about Tarrlok's fate and start beating herself up for it, just as Aang was morning everyone's deaths while he had to defeat the fire nation?
Why should Korra just for randomly decide to go into the spirit world and start talking to her past lives? Clearly she hasn't worked on her spiritual development at all in the six month gap; an issue/conflict would have to accrue to give her a reason. Starting with going into the spirit world to seek help looking for Amon and Tarrlok from a spirit. A scary spirit would be most appropriate to tell or even show Korra what happened to them, in fact it might even be Koh he said to Aang "we'll met again" and he never did so could have just meant the avatar in general- it would also be a an awesome way to re-explain what the spirits can do. After finding out she failed to protect someone she would want advice on what to do leading her to talk to her past lives. Don't ask Ruko or Koshi…
Mike and Bryan said they weren't going to hit the reset button in book 2 and would apply the stuff Korra learned from book 1. In the book 1 finally Korra learned to be selfless, finding out about Tarrlok's death would be showing the progress she made because she would be feeling someone else's loss. The "reset button" is often used is because of "status quo is god" (when stories keep everything the same throughout the series making it very stale). Not treating Tarrlok's death like its important in book 2 would be "status quo is god" because despite the fact that a major character with a very important job as councilmen of Republic City is now deceased everything stays the same?!
If Korra doesn't find out she will believe that life has no consequences. In the first episode of Adventure time at the end it made fun of the poor execution of lessons in other kid shows by having the main character Finn say "if I break a royal promise I get to throw sleep overs, fight zombies and reverse death itself!!!" That will pretty much be Korra's outlook on the world when allowing the bad guys to get away.
It makes sense to explore this issue again next season to go along with continuing maturity of the show given it is a dark and complex issue.
I think it would be a fantastic plot twist because most people aren't excepting Korra will be able to find out, think it won't matter if she doesn't, or this idea never even occurring to them.
It will also be a good slapping in the face of the idiots who think they are still alive. (Did Amon blood-bend their brain before he left?) After 9 months people are still holding onto the idea that Amon and Tarrlok could have survived the explosion backing it up with BS arguments and there is no word for how much it pisses me off. Jet was murdered by Long Fang in book 2: earth and 6 years later there are people who are still people who think he is alive and I have no idea why, he never showed up again so why can't these idiots put two and two together? If Tarrlok's death doesn't contribute to the rest of the plot we'll never hear the end of it!
So let's face it, whether Korra makes the discovery of Tarrlok's death or doesn't is going to make or break The Legend of Korra and forget killing two birds with one stone; this would kill the entire flock.
"…The avatar must experience sadness, anger, joy, and happiness. By feeling all these emotions, it helps you understand how precious human life is, so you will do anything to protect it" – Avatar Yangchen
"Suffering will be your teacher" - Fire Lord Ozai