Like Keweenaw says, nights could be 40, could be 60. If you're near the big lake, well, it's still quite cool in July. A light breeze can drop the temp 10-15 degrees, easy. I've always brought a bag, and always seem to be snuggled into it.
Shoes - I only wear them once at camp and the pack is off. For multi-day backpacking, you really should be using a good hiking boot, and have them BROKEN IN. I don't think it has to go too high on the calf; but certainly ankle protection. Beyond just the rocky trail, there will be places where you're walking in understory and might not be able to see the trail below your feet. Between all the roots and rocks, you don't want to twist an ankle. As for the water, most of the crossings will typically also be over a boot in depth - More like a knee or waist, in places. However, there are logs and rocks that, if you're lucky, are above the water, or at least only minimally below. A good sole goes a long way on the slippery stuff...
Hugginin - One of those "I'm going to get there one of these trips" campsite. Thought about it last year when we did the Minong just like you will be doing; but visions of dry shelter and a sandwich from the store danced in our heads and we elected to enjoy the last night in comfort at Washington Creek.
Ferry - Like Bob said, I don't think anyone is getting rich offering the service. As a concessionaire for NPS, they have to agree to plenty (set routes, mail service, etc) I'm sure. Not unlike the utility surcharge (21%?), Isle Royale is just not as inexpensive of an experience as other parks.
HOWEVER, it's worth every cent!
