Your paint stripe idea is a good one.. I never thought to try that.. but the stripe will just show *contact* pattern - not so much a wear pattern and your back side contact pattern that matches your flat idler appears normal to me You'll get the same result even if you replace the flat idler with a new one
The paint stripe, probably should reveal if there was belt slap or rubbing/contact that should not be happening and as you have stated the contact pattern looks normal on the inside too , all that is left is to run the belt as usual and check after every use for any unusual wear pattern forming. but I'd say it looks as if you have the problem mostly resolved.
(and the back edge KE4 refers to would be expected with the belt tension flexing the belt ever so slightly as it passes through the V groove, the corners will tend to "roll" a little as they fit in a pulley that is deeper than the belt )
How poor are they who have not patience. What wound did ever heal, but by degrees? - Iago (Othello Act II, Scene 3)