Hoping for some wisdom on flat spots in ropes...Is the "keyhole" test the best way to know when to retire a rope? (Where you fold the rope back on itself and see if it can be pinched into a V or leaves a keyhole shaped loop). Or is there some more reliable way to know if the rope core is okay or if it's damaged?
Do you still have confidence and trust your life in that rope?
Rope is cheap as far as safety equipment goes. Imo its an easy decision, but I have a few quid spare on occasion so maybe less of an issue to chop the offending areas, buy a new longer one and carry on.
I also find belay device choice can affect rope flattening.
Just run it through your hands, paying attention for severe sheath damage, irregularities and flat spots, e.g. as you mention, with a v bend instead of a keyhole shape being a warning sign. It is a reliable method. If you find something of concern, cut it out of the rope.
Rather than retire a whole rope, I tend to remove the knackered bits, which leaves me either with a useful short rope or a useful bit of tat. Damage tends to accumulate toward the ends of the rope unless some particularly traumatic snag/fall occurs.
Of the various soft/flatspots I've cut into over the years I've never seen any core damage, in my experience at least it's just a slight excess of sheath cause by slippage.
Hard not to wonder about them though when you have one between you, your failing arms and the scree below!