Yeah, honestly he's like a slightly lesser Surfer. He can essentially do everything that Surfer can just on a slightly lower level, and he has cosmic fire on top of that. Any herald of Galactus could beat any HST character. Some, the entire HST.
I think if L had confirmation of Johann's existence beforehand, with effort, time and a lot of planning I think he could eventually catch up to Johann (but maybe not subdue as Johann isn't above killing himself). However if Johann were a myth and nobody like Tenma were around telling people, I don't think even L could take him down. Johann's biggest advantage was the fact that he was able to keep himself as a rumor or myth to the majority of those involved in his case.
Goku withe the IG!? The infinity Gauntlet (wielded by Thanos) soloed over a dozen Galactus level beings including Galactus himself. I don't even know Chakravartin but I know enough to say Galactus loses here.
It is. The first hit is meaningless without the second. Not like a normal poison. Not to mention it completely erases the opponent leaving behind a her suzumebachi crest symbol. I don't know of any normal poison that does that. Lol.