Theoretical Question - Challenger v Diamond

I'm an iron and I don't have enough experience to know.

Who would win between a Challenger Morde v Diamond Vayne (Counterpick)?

Does the counterpick really affect it that much?

Straight 1v1.

Who wins:

Pre-6

Post-6

And who wins if it's not a 1v1, and it's a normal lobby with a Challenger Morde v Diamond Vayne top lane.