I think Zoro would kill Buggy if he used Haki against him. I don’t think that much if anything has changed when it comes to Buggy’s power level since East Blue.
I can see an admiral getting killed to get more cross guild points. They’re strong enough to be taken seriously but weak enough to where Yonko level characters can kill then easily.
I actually would have preferred Crocodile to become a Yonkou instead of Buggy. Both were classic One Piece villains but I could take Crocodile far more seriously.
Usopp learning Haki in Dressrosa is a big one. It arguably saved him from running away from Sugar after she turned Robin into a doll.
Chopper walking into the Franky house with the Monster Trio after the four of them saw Usopp defeated and fighting the Franky family is another big one.
I will...
I believe that SSJ3 can be suppressed, but I also imagine it is even more straining than 100% Freeza, or Grade 2 or Grade 3 for that matter, so it must be more difficult to do so.