so is the spacecraft already exposed? to save weight maybe?
No need to try to accelerate extra mass if it won't do any good.

Once there's no appreciable atmosphere around, they can drop the shell.
I wonder how they calculated the trajectory with all the gravitational forces and what not in the solar system. It's not like going to the moon so they must have a lot of burns to keep the giant penis on track.
Nothing they haven't done before. Cassini did 2 flybys of Venus and 1 flyby of Earth before it picked up enough speed to get to Jupiter in a reasonable amount of time, and from there it was flung to Saturn. They didn't just do a direct route because it was a damned massive probe, at least as far as these things are concerned. A direct launch would've caused it to take much longer to get there, assuming it would make it at all.
And back in the olden days, they managed to figure out the trajectory for Voyager 2, such that it was able to do flybys of Jupiter, Saturn, Uranus, and Neptune.