Did Ned fail to notice Renly as a major supporter of his?

[Ghost+Nymeria4Eva]

Rereading GoT, I noticed the story of the locklet. 

Quote

Ned was not sure what to make of Renly, with all his friendly ways and easy smiles. A few days past, he had taken Ned aside to show him an exquisite rose gold locklet. Inside was a miniature painted in the vivid Myrish style, of a lovely young girl with doe’s eyes and a cascade of soft brown hair. Renly had seemed anxious to know if the girl reminded him of anyone, and when Ned had no answer but a shrug, he had seemed disappointed. The maid was Loras Tyrell’s sister Margaery, he’d confessed, but there were those who said she looked like Lyanna. “No,” Ned had told him, bemused. Could it be that Lord Renly, who looked so like a young Robert, had conceived a passion for a girl he fancied to be a young Lyanna? That struck him as more than passing queer.

And later we learn of a letter Loras has written to Highgarden telling them to send Margery to court so they could have a new queen. It would have seemed really ridiculous at the time considering there was Cersei with three kids. But Renly probably knew about the incest and was planning to get rid of Cersei and install a new proper queen before Ned's death. Ned seems not to notice that Renly could have been a major supporter in his cause against the Lannisters. Anyone else think so? It seems like a major missed opportunity. 

[TMIFairy]

Ned's whole life is a string of missed opportunities ... he was a good boy, but not very bright.

[A wilding]

Yes, as you say Varys tells Illyrio in AGoT that this is Renly and Loras'es plan.

Though it is doubtful that Renly knew about the incest. He tells Stannis at their parley in ACoK that the incest story is a superb piece of propaganda, but that he (Renly) does not believe it.

My pet theory about that conversation between Renly and Ned is that Renly knew perfectly well that the resemblance between Lyanna and Margaery was slight, but was delicately sounding out Ned to see if Ned could be persuaded to join in the plan. Renly had to be careful as he did not want the Lannisters to find out too soon and perhaps strike against him first. However when Ned acted dumb and did not seem to get the point Renly thought it best to back off.