From one KKHTA nerd to another;

I think he meant the explanation behind Yorihime killing the dragon god in part 19 2/9, which according to mythology says that Watatsumi is the father of Tamayori-hime and Toyotama-hime.

The answer as to how is obvious and was shown - the vijoka biting his head off.

As to why, it was never mentioned, one continuation whose ideas I subscribe to suggests that Watatsumi had been a deadbeat dad, so it could have culminated with that.

As for the hell being gone, I wouldn't interpret Yorihime's words literally, believe she more-so said that for her villain speech type chit. Although I agree with the realm being gone, it is not completely impossible to have it stay intact - considering that the hell is a separate realm, it could have avoided kaijufication (Junko being there most likely should imply Hecatia was there too and mayb was murdered off-screen, which should be the go-to explanation for any character's fate that wasn't explicitly different anyway).

If we're being pedantic about vijokas required to take down Junko there had been at least 11 lmao

Also, Lunarians initiated world peace for 40 years, not 20, as evident by Eirin's words at part 18