After reading your experience yesterday, I consulted a friend who has also done a lot of candying of citrus and other fruits. She lives in Vista and has several kumquat trees and several rare and unusual citrus.
She can't figure it out either. She suggested trying a very small batch, parboiling them briefly in a couple of changes of clear water - as one would do with citrus peel, then go through the candying process with three times the amount of syrup as fruit.
Her regular process is virtually identical to mine except she adds half a cup of light Karo syrup to 2 quarts of sugar syrup. I can't see how that small an amount could have much effect. I have never used it, mainly because I never have it on hand.
I am going to stop at the middle eastern market on my way home and buy some kumquats and go through my usual process and see if I have any problems.
I am really stumped by this.
I am also going to see if my microwave method will work on kumquats. I have never tried it with anything except orange, lemon and grapefruit peel.
I thought the parboiling for citrus peels was to remove the bitterness in the orange or grapefruit rinds. I would have thought it was unnecessary for kumquats, where the rind is sweet. I wonder whether it might have been better NOT to "core" the fruit before beginning to cook it. It seemed to me that that small hole in the kumquat enabled the juice to quickly "leak" out , at which point the kumquats "deflated". If they weren't pierced, maybe the sugar solution would slowly go into the fruit, and they would not collapse as they were cooking. I also couldn't see how a covered pot would enable the sugar solution to get more concentrated with time. The kumquat juice and condensing steam off the crockpot lid just seemed to further dilute the cooking liquid. How about cooking on low, without the lid, replenishing the sugar solution only as needed to keep the kumquats submerged?