No, fructose is converted to fat. IIRC it can not be used for anything else.

Gluconeogenesis is both effective and has nothing to do with a person being skinny. It takes time to ramp it up but once adapted you see no difference* in case of short workouts and improvements in marathons and such.

For some examples check Anthony Chaffee and Shawn Baker who are carnivores and absolutely not skinny. Anthony also made a bunch of interviews with various athletes and body builders.

* Actually I experience one difference I've never seen anyone talk about: I had constant nausea when squatting before carnivore. Since carnivore I didn't have a single instance of it and I increased the weight by 10kg. I guess it may be just me.