The quick answer, in this context is that a single issue of an ongoing story arc is a chapter. The reason we mention chapters is avoid confusion and anyone saying "Well, this story goes across six issues, so I can post two issues in their entirety." It, of course, doesn't work like that.
My assumption is fanzines would fall under the same remit as fanart. It's not really what this community's for, but fanart's posted on occasion if accompanied by a legal scan and the artist is credited. If available free, then the rules for freely distributed works apply. If there's a cover charge, then I think it's only fair to respect the rights of those that produce it and not post the entirety of the work. More so than normally, really. There's no reason for us to be snatching crumbs out of the mouths of other fans.