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:
- 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.
- 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.
- 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.