Ya, you should swing by the space station and pick up your lead shielding on the way out, drop it back off on the way home.

Iss is in the wrong orbit for that.

Something could be put in the right orbit for it?

Couldn't the shielding be left with the ISS?

Good point, and would that would take care of the gravity well aspect of the issue. You'd still have the same amount of mass to accelerate, though. I suppose that wouldn't be as much of an issue once you weren't fighting Earth's gravity as well as inertia.

It doesn't. However, the more launches required to assemble your spacecraft, the more chances there are for something to go wrong.

That's a silly argument. You could just as well say that the more things you launch that aren't a passenger-loaded vehicle, the more likely you'll have worked out the glitches by the time large numbers of lives depend on it.

A mission to Mars is guaranteed to be the most complex space mission ever attempted. Every additional rocket you need for the mission adds complexity. Complexity is the enemy of reliability.

What happens to your Mars departure window when the rocket carrying your life support system goes off course and is destroyed by the range safety officer?