Frames of reference
The ship doesn't fly into the blast since the blast and ship are moving at the same speed when the thing goes off. What does happen is that the ship accelerates into the blast zone as it is being pulled by the mag-sail. But the plasma is doing a good fraction of lightspeed - 5% or so I hope else it's pointless as a drive - so it disperses very, very quickly. The prompt x-ray flash is about the only impinging radiation on the ship and that's pretty straightforward to shield against.