What do you think would have happened had Renly secretly requested Ned’s help in replacing Cersei with Margaret?

Good question!

I think it would depend on how Renly approached Ned, and given how he approached Ned in OTL, the odds are that Renly would botch it.

See, the thing is that coming up to Ned and saying “I don’t like the Lannisters, I have a scheme to replace them with the Tyrells” is the worst possible pitch. Ned doesn’t like this kind of politics at all and would probably refuse to participate.

However, if Renly came up to Ned and told him about the incest and that the Lannisters have to be removed before they kill Robert and seize the throne, I think Ned would go along with that. He would probably be somewhat disillusioned afterwards when Robert married Margaery and the Tyrells replaced the Lannisters and the whole game kept going as before, but in the moment he would respond to a call to save the realm.