Couldn't you prove/disprove things systimatically?
For instance you have shown that it's not the yeast by repitching, how have you proven that it hasn't simply run out of sugar? couldn't you take some of your batch into a smaller vessel and add more sugars to see if you get fermentation? yopu would only need 500ml or so in a sanitised jar without adding extra yeast, if after a few days you get nothing then try extra yeast in the jar. You need to take a scientific approach to it.