The posited exit trajectory was for a single body. As the break-up was mainly caused by tidal stress on weakly bonded chunks, the trajectories will be somewhat different. Results in a ellipsoidal spray pattern.
Orbit is still valid.
Example: shoemaker-levy 9.
The pieces continued along the parent body orbit.
Ison is doing the same thing.
Plus what is left won’t come closer than .43 au from earth in January.
No, if there are any pieces broken off they will continue along the same route as ISON.