Thanks for playing, I'm ecstatic you like it so much!
Z used to be the button to continue, but some people cominted they were continuing without even knowing what happened, so I thought it'd make more sense (if a little annoying) to move it to the start button so you atleast have to make conscious thought about it. Full screen will scale to whatever your monitor resolution is, so only even scales of 640x360 (720p,1080p,etc) will escape being all strechy and weird. I'm looking into black bar options to keep it pixel perfect in full screen, but for now stick with windowed.
Also, the guitar is real! It's got a vst amp/pedals and some post processing effects that make it sound sorta synthy.