As for the first issue yes, currently, you have to select a plugin editor to be using. I do plan on making the editor activate itself at startup - replacing any other loaded editor. Now, in a real world usage, you should only have one editor plugin that is best suited for your project. So, this shouldn't be a problem. If however, you need features from several plugins at the same time, then which ever plugin sets itself active will just be the default one.
I planned to have a more explicit setting of this (and probably will in the future) - via the tools -> plugin manager (which is not implemented at all right now).
As for issue number 2. I don't know why it doesn't work for you. Here, it works fine. Start editor - select WGE Editor (not WGE SceneViewer), then load a project (or load project first), then open a scene. I can pick (which means that it is active).