All modes are using a shared key-map for the ease of defining new keys.
i'm not sure what you mean by this.
slime-repl-mode does this: (use-local-map slime-repl-mode-map)
but somehow (my guess is precedence and/or setup order issue) slime-editing-map gets used instead of slime-repl-mode-map.
Maybe it's better to have a function which will define a keybinding for all relevant slime modes?
imho that would lead far and duplicate the logic keymaps, modes and their inheritance provide in emacs.
the current setup with a tree of keymaps inheriting from each other is flexible enough. it's just the repl mode seems to misbehave for some reason...