Ned was a fukking idiot. Even after all that shyt had he just chosen Rhenly everything would’ve been different.
You're saying all he had to do was something he wouldn't possibly have done due to who he is as a man/character though. Renly had no claim to the throne, it would have sparked a war and I'd imagine at some point Ned would still find himself getting betrayed by Littlefinger.
Robert's injury/death was a checkmate move, The only potential thing Ned could have done is said nothing to Cersei/Littlefinger, and either sail to Dragonstone or flee to the north. Sailing to Dragonstone would prob be the best bet since it would be easier for Lannister solders to intercept him on foot/horseback. So let's say he got to Dragonstone, warned Stannis, pledged the north to him, and then sailed to Winterfell. The wrench in the plan would be Melisandre though. She was already on Dragonstone poisoning Stannis before S1/Book One ends. Would Stannis have Ned arrested or executed for not pledging himself to R'hllor/Lord Of Light? Would he compromise in order to secure the north? Who knows but I'm leaning towards him choosing Melisandre over Ned.