Yes, that code edit I added was added using the customizer (I almost always use the customizer for this kind of edits – is that ill advised?). Your solution for the navigation worked perfectly! Thank you!
As for my logo – yes, certainly I could upload it using the customizer/”header image” but what I actually want is to add an image to the left of the current site title – so to display both the logo AND the site title. Whereas if I upload a logo through the customizer it REPLACES the site title – I can’t figure out how to display both at the same time.