Can I boost my sound even more, after adjusting my audio levels in FX?

You can Extract your audio within iMovie and then use an external audio editor to boost the volume. (right click the iMovie "file" and Show Package Contents. Look for your audio file in the Media folder) This audio file can be imported back into iMovie.

Another option (I've convinced myself this works, but perhaps only the logic of it works). Extract the audio twice (once for each audio track) and then go back to the original track and reset the volume to max (when you extract audio, it essentially makes a copy of the audio file and sets the original footage volume to zero). This gives you three sound sources, which has to be better than one...

Re: your silly question -- sorry, that part of the iMovie interface isn't adjustable that I'm aware of. But, when you use the external editor, it's likely to have a larger waveform display.
John