Hi Fishy Mama, I think papabear has given a very good explanation on your queries.
I hope I can give my best knowledge for opinion here

. High water temperature does play an importance role in dissapearing hi but never heard about sumi. Sometime, the koi must have gone thru a very stress period for some reason and lost the Hi. If the hi lost due to stress, I doubt if the Hi will recover. I have heard of showa lose its hi but not all. There was still remnants of hi, under a proper care by the owner and the hi did recover. But that could be once in the blue moon. Seldom heard of sumi lost unless secondary sumi and it will hardly return. Some expert here should be able to advise you.