Tywin would absolutely have refused to believe him, and would have retaliated harshly against Tyrion for trying to bring the proud name of House Lannister into disrepute. 

If Tywin had known, the assassination attempts would have been more professional and far less sloppy than they were. 

I don’t think Jaime knew, because if he knew Cersei wanted Robert dead, he would have done it himself. 

opinions-about-tiaras:

racefortheironthrone:

I don’t really understand what you’re saying here: you seem to be suggesting that replacing Cersei with Margaery would somehow avoid war, but would involve Stannis going to war with the Lannisters; Cersei being executed but somehow not for incest and adultery.  

Let me clarify my previous post:

Renly’s plan doesn’t work if he doesn’t know about the incest. If Cersei is a faithful wife, it doesn’t matter whether Robert sets her aside in favor of Margaery, because Cersei’s children will inherit ahead of Margaery’s children. And if Margaery’s children aren’t going to inherit, Mace Tyrell – who is consistently motivated by a desire to put a Tyrell grandchild on the Iron Throne whether the father is Renly, Joffrey, or Tommen – has no reason to go along with the plan. So Renly had to know, because the plan did go ahead, so he must have persuaded Mace to go along with it, and there’s really only trump card to play in that argument. 

To answer your specific points:

1. While Robert is fairly young, he’s not in great health, and could die at any moment (in part because Cersei is repeatedly trying to assassinate him). But Robert living or dying is only relevant to the Tyrells if he marries Margaery and sires an heir with her. If he doesn’t marry, or marries someone else, they (and Renly) are just as out of power as they would be if he stayed married to Cersei. They only have a motive to reveal the truth if that means that Joffrey, Tommen, and Myrcella are disinherited and Margaery’s children take precedence in the succession, and that scenario requires Robert to agree to marry Margaery first. 
2. Robert replacing Cersei with Margaery doesn’t actually prevent war, as even your scenario points out, because replacing Cersei also means replacing her kids, and Tywin will go to war to uphold their rights to the succession – and if there’s no legal justification for Cersei and her children being set aside (like say incest and adultery), a lot of other people might join Tywin, just like in the Dance of the Dragons. 
3. Stannis left for Dragonstone when Jon Arryn died, before Renly had a chance to put his Margaery plan into effect. Hence why Renly’s only at the stage of showing Margaery’s miniature to Ned to gauge whether he can use the resemblance to Lyanna to entice Robert in Eddard VI, and talking up Margaery to Robert in Eddard VII. He hasn’t yet brought her to court so that Robert can bed her and then marry her by the time that Robert dies. So it’s not possible to “quietly kill Stannis (Tyrell style) if Robert refused to marry,” because Stannis has already taken steps to protect himself. 

I will toss one thing out here; if you have a VERY low opinion of Renly and the Tyrells, you can make “Renly doesn’t know about the incest” work if you simply assume Renly is planning to have Joffrey, Tommen, and Myrcella murdered, thus neatly removing them from the line of succession in a way that doesn’t give a shit about their legitimacy.

This is also highly implausible, mind you, and would be politically very stupid, and doesn’t meet Occam’s Razor, but still. There you go.

I think the main problem with “Renly as multiple child murderer” is that, while that might work if the only Tyrell that Renly was working with was Olenna, who’s a cool enough customer to dispatch inconvenient teenagers, and maybe even Garlan or Margaery (depending on how much those two knew about the Purple Wedding; and even then, I’d lean more toward Garlan than Margaery). But Loras and Mace are far too conventional in their morality to go along with that plan.

I think there are two main, complementary explanations to how Renly figured it out:

1. Renly was a political actor in the royal court. One of the main activities of someone in his position is to closely observe and/or spy on the other main political actors. Given that Jaime and Cersei are so indiscrete that Varys, Pycelle, Littlefinger, Stannis, and Jon Arryn all found out, it was eminently possible for Renly to have found out by either observing/spying on Jaime and Cersei directly, or by observing/spying on Varys, Pycelle, Littlefinger, Stannis, and/or Jon Arryn and finding out what they found out. 
2. Renly knows what Robert’s kids look like. Edric Storm is Renly’s ward and living in his own castle, and he looks like the spitting image of Robert Baratheon. Moreover, if Renly is spying on the other political actors, he might well find out about Stannis and Jon Arryn investigating Robert’s other bastards in King Landing, or Jon Arryn inquiring about Pycelle’s book, or Littlefinger whispering about Robert’s kids not looking like Robert, etc. Once you’re there, it’s not hard to put two-and-two together.

If it’s before, Sansa’s identity plays right into Bronze Yohn’s efforts to “deman[d] that I call my banners and go to war.” Here is a Stark at hand, a symbol to rally around, a fair and noble lady who can tell everyone about how Joffrey and Cersei are monsters and murderers – as Bronze Yohn whispers about how all of this proves that Jon Arryn was murdered by the Lannisters – and plead with the upright and honorable lords of the Vale to help her retake her inheritance from the treacherous Boltons and then topple the evil Lannisters once and for all. 

If it’s after, it’s complicated by the fact that she’s now both a witness and a suspect in the murder of Lysa Arryn, and that she’s under the protection and lock and key of Petyr Baelish. Now it becomes about Petyr Baelish the murderous Svengali, the man who murders his wives and kidnaps fair maidens as part of his corrupt agenda, and so on. 

It very much depends on status:

“One more thing. A trifling matter.” He gave her an apologetic smile and told her of a puppet show that had recently become popular amongst the city’s smallfolk; a puppet show wherein the kingdom of the beasts was ruled by a pride of haughty lions. “The puppet lions grow greedy and arrogant as this treasonous tale proceeds, until they begin to devour their own subjects. When the noble stag makes objection, the lions devour him as well, and roar that it is their right as the mightiest of beasts.”

“And is that the end of it?” Cersei asked, amused. Looked at in the right light, it could be seen as a salutary lesson.

“No, Your Grace. At the end a dragon hatches from an egg and devours all of the lions.”

The ending took the puppet show from simple insolence to treason. “Witless fools. Only cretins would hazard their heads upon a wooden dragon.” She considered a moment. “Send some of your whisperers to these shows and make note of who attends. If any of them should be men of note, I would know their names.”

“What will be done with them, if I may be so bold?”

“Any men of substance shall be fined. Half their worth should be sufficient to teach them a sharp lesson and refill our coffers, without quite ruining them. Those too poor to pay can lose an eye, for watching treason. For the puppeteers, the axe.”

“There are four. Perhaps Your Grace might allow me two of them for mine own purposes. A woman would be especially …”

With a warhammer. 

See here.