Were there any award flights available into Blenheim? That (apart from being a highly glamorous location ) is only about 1.5 hours drive from Nelson, and it cuts out the most narrow and twisty section from Picton to Havelock.

If I were you, however, I'd buy an air ticket. After all that travel you really won't be feeling like dealing with ferry and driving, however nice the trip might be under ideal circumstances.