Best advice - rent a light canoe. Leave the W/C home. (I'll never take my W/C back to the BW, just too heavy)
August on a river, expect low water, never been on the Isabella so can't help there.
Yes, in most areas, you can only legally camp on established sites. If a "ranger" checks you, you will be in trouble.
Generally speaking, the established sites are there because that's where the good places to camp are.
The easiest routes would be larger lakes with few portages, Seagul-Sag, Moose-Basswood, Sag-Knife.
5 days - depends on how far/long you paddle, could be 100 miles or 20 miles, just depends.
(my answers are biased as I like large lakes and clear water, not a fan of rivers (small water and stained water))
(small water usually means plenty of portages)
My fav area is the Knife Lake area, nice water and very clear.
Dan
oh, saw your post on the cart string - they are not legal in the BW, at least not most areas, there are a handful (3-4) of portages to motor lakes where they could be used. And most portages are way too rough for them to be useful anyway. They would be just another item to carry.