Thanks for taking interest in this topic and asking those valid questions.
1) I haven't deviated too far from the OOTB Harness setup. It's using the Header Left screen layout and hiding the header involves me setting the header's panel height to 0 from the default value of 40 px.
2) The left panel is what is being toggled with the navigation icon.
3) Before I asked this question, i searched around to see if there were previous discussions on the topic and i didn't find anything that really matched what i was trying to do or even came close to it for the matter. Can you help me understand how exactly a layout group would help?