Only in Homestuck would the author threaten to kill himself off.
I bet he's gonna do it too. He can't just present the possibility of killing someone and then not take it.
And hey, I dunno, Slick's pretty canon. He's just sort of.. become a joke. xD
I have a feeling he probably won't die. He's a pretty tenacious guy, and he was one of the biggest parts in allowing Lord English to appear in the first place. Though I guess English probably doesn't care about his pawns all that much..
But still. Slick at the very least is a pretty darn important guy. Alternia's version of Jack Noir, and becoming a kind of mirror to the bunny? He's a plot device if there ever was one. o: He's served as one a heck of a lot so far, and I bet he'll probably continue to.
I think regarding Eridan and Feferi, like Karkat said, these are probably just be a couple of the dead trolls they keep meeting up with that are from offshoot timelines. You probably can't exactly hit god-tier while both of your bodies are dead, after all. O: I bet these guys hit god-tier and then died somehow. Like that dead god-tier Karkat that appeared in one of the Flash pages.
Oh, oh. Also to do with the dream bubbles. Feferi reviving a Wayward Vagabond aaaaaa. Not sure if this is the alpha timeline one or not but at the very least he'll get to be adorable for a few pages~