Your body will use all sorts of different substrates to make glucose in gluconeogenesis, i.e. lactate, amino acids, the glycerol backbone of a triacylgycerol (but not the fatty acid itself). Gluconeogenesis will proceed and replenish relevant glycogen stores, albeit very slowly. Depending on how high of volume or high intensity your training is (and how well you can run on ketones/fatty acids) this may be a problem for training or it may not.
Well it is metabolized (or can be anyway) by skeletal muscle, just not at nearly the rate and load that the liver can handle. I don't particularly like super huge amounts of fruit for reasons I've stated elsewhere and if your fructose intake is really high, that may be hindering your gains. I'd lean more towards rice, potatoes, carb supplement, and oats if you're looking for performance.
Additionally, if you're number 1 goal is to just get lean, then going full on ketard with carloric accountability is probably your best bet with a refeed every 7-10 days or so on the order of 300-400g carbs.
I'd pick up Medical Biochemistry : Human Metabolism in Health and Disease as a nice primer. Also Advanced Nutrition and Human Metabolism is good too.