From JohnH's trip report, those portages should be clear of deadfall, so good.
The portage between Tobin and Duncan is the biggest obstacle, but take your time and it will be fine. Assume it will be hell on earth, but it will be worth it, and when you're done it will not seem
that bad, but feel like an accomplishment. It has taken me anywhere from 1.5 hrs (solo, overnight pack only) to 3+ hrs (family of 4). I always portage in 2 trips.
Once in Duncan Bay, you can camp at Duncan Narrows or the Duncan Bay CG at the end of the bay. I think the narrows is a little more scenic, but there may be more company with boaters who don't want to go all the way to the end. However, the last time I was there we had the place to ourselves, so you never know.
From Duncan Bay to Belle Isle, 3 hrs is a reasonable estimate. My daughter and I did it in 2.5 hrs taking our time, but skipping a portage as the big lake was calm. As JohnH says, those 2 portages and short and easy. From Lane Cove to Belle Isle you can round the western point of Lane Cove and duck behind Johnson Island for protection if need be, or just to make the paddling easier. If it's too bad, Lane Cove is also nice and pretty protected to stay at. But Belle Isle is my favorite place on I.R.
If you get there, get up early for the sunrise--it can be spectacular!
Here's a trip report and pics from my last trip to that area, although we started at McCargoe
http://moskeybasin.com/IR2009/Main.html
Have a great trip!
John I