Yea, what you're missing is that he only went to KL to talk to Cersei because she sent for him. He did NOT want to go back there and he was really nervous because of his involvement in the murder of her son. Turns out, she didn't know he was involved and he basically just gave her a spin job on what he's up to and why he needs to leave KL again to go back to the North. Remember he also spilled the beans to Olenna about Cersei and Lancel so that Cersei could get locked up by the High Sparrow.
Littlefinger is NOT waiting around for some kind of decree from Cersei to legitimize his claim to the north. He's much more politically savvy than that. He wants to marry Sansa and take over the Stark family lineage in Winterfell.
Littlefinger is NOT waiting around for some kind of decree from Cersei to legitimize his claim to the north. He's much more politically savvy than that. He wants to marry Sansa and take over the Stark family lineage in Winterfell.