1) The shadow is that of the lens. It is more than LENGTH contributing to the shadow; FOCAL LENGTH or FIELD OF VIEW would play a part (suspect that's the reason here, rather than length), as would the diameter of the lens and the height of the built-in flash above the lens barrel (relative to each other). I would find it very hard to produce a shadow with a long, long, long 600mm and using a super powerful torch mounted where the flash is, for example (assuming the beam of light is not obstructed by lens barrel).
2) An external flash is one solution, because it raises the light source significantly higher above the lens barrel and thus would avoid this problem. There are other possible solutions, but they tend to be fiddly. Alternatively, just don't shoot WA lor.