As soon as the IC has moved more than 2" from the unit then it is no longer part of the unit and is not restricted by their movement.
Note that joining happens at the end of the phase, while leaving occurs as soon as an IC moves out of coherency at any point in the movement phase. If the model's full movement has not been expended to achieve this then of course the remainder can still be used because it is not bound by the restrictions of the unit it has already left.
It doesn't say as soon as they're outside coherency. It might mean that who knows.
An independent character can leave a unit during the movement phase by moving out of coherency with it.
Does that mean you need to have completed your move or just any time during your move, it's unclear. The only indicator is that everywhere else we see move, it means the whole move, start to end. Some rules, like tank shocking have specific rules for when things happen along their move.
For example, moving through cover affects the whole move, if you'r 5" away from cover and you attempt to move into it you don't walk forward 5" then take a test, you take a test an if you roll a 3 then you can only go 3" despite the fact that gets you nowhere. In that case move means the whole movement start to end.
With that being the case I would assume moving out of coherency is the full movement of the IC. You measure, you pick him up you put him down outside of coherency. When you start the move you are in the unit, you are in fact moving that unit at this point, so you are bound by the restrictions of it. If the unit is in cover for instance you might not be able to even move far enough to get away if you roll badly. You work out far far you move at the start of your movement, not during your movement.
Aramoro