1 - What do you call the leaders of greatest pirate organizations? Right, a Yonko. He is the same thing, whether he was called that or not.
Same for Roger, despite never being called a Yonko, he was the same.
2 - Because nobody massacres nobody in Oda's story unless he wants it. Until today nobody massacred an Admiral either but today Oda decided to feed an Admiral for the same of Pirate's hype.
3 - Those moments are far more rare than what you think. Same as >Yonkos constantly engaged in fights with each other and then we find out they haven't seen each other in decades...
4 - Akainu barely pulled through as a winner, lol while this implies Xebec ate an Admiral for breakfast. Despite all the bad showing BM had, it wasn't rookies and no aCOC clowns who defeated her, it was a bomb. Kaido ran a gauntlet that no other will ever run, he is clean.
5 - Sure and as I said, those fights are far more rare than people realize.
6 - It is self-explanatory.