Who actually tests their light before they need it? I had a cheap keychain one that worked fine for months, then died the second I was in a dark stairwell. Now I just keep a tiny AAA light on my keys and check it once a week (takes two seconds, way better than feeling around in the dark). The phone screen thing works but it kills your battery fast and it is not bright enough to see steps. A cheap $12 light with a real switch beats a fancy one with a button that gets bumped on in your pocket.

A 4 inch planer probably worked fine on that berber but I've seen guys take that same trick to a glue down vinyl plank job and now you got a dip nobody can hide. I'm not saying don't ever touch the subfloor but the hump is usually telling you something, like a joist crown or the slab heaving, and planing it down just hides the problem until it moves again. Patch and float gets a bad name because people sling it thin and don't let it cure, but if you mix it right and feather it past 18 inches you get a fix that moves with the floor instead of fighting it. That retired couple might call you back in 8 months when the planer dip telegraphs through the backing. I've pulled up enough of my own work to know that the fast fix is usually the one that bites you later.