Was comet ISON in a Kepler orbit prior to disintegrating? If so, it is possible over time that Kepler orbits can be altered by increases or decreases in the mass of of celestial bodies? For instance if ISON was in a Kepler orbit and it lost mass did it maintain its orbit and disintegrate in its orbital track or does it alter its track due to the loss of mass?
To your first question, yes, it was in a Kepler orbit, (this interactive model shows just how it appeared). ISON never completed a full circuit around the Sun since this was it's first time coming out of the Oort Cloud and it disintegrated.
Further answering, the orbit of a body around another in space is not affected by it's mass. Remember the experiment that proves two object fall at the same speed regardless of it's mass (here the experiment is made on the moon with a feather and a hammer) and remember that orbiting is just falling at a certain speed and angle in respect to another body.