While the current coming out of Russell into the Narrows can be very bad, it is likely to be much less so in August. Still, I do recommend taking the portage. Thirty years ago in May, my brother & I had to pull some paddlers out of the post-ice-out torrent there... and we only just HAPPENED by, having spent the prior week further east in the Cache Lake area. Those guys were the first folks we had seen in many days and we had to get a fire going to fend off the hypothermia from their blue bodies. That area is trickier than it seems almost any time of year.
Re: Russell camping. I have found it "hit & miss". Some years, no matter what the season, the area is packed. Other times I expect it to be packed and we see nary a soul. Good luck. I'd go for it as Russell is pretty as is the Poet Chain to the east.
Some folks like to camp within earshot of the Falls, saying "it puts them to sleep at night." Others detest the noise & say it keeps them from sleeping. You CAN hear it a long ways off, especially when the wind is blowing in the right direction. If you find the sound bothersome, put an island or two between you & the Falls. Me? I like hearing them at night.
The portage into McDougall is one of the easiest in the park. The lake is well worth the paddle, especially if you're fishing for smallmouth bass.
All Quetico paddlers would do well to do the Poet Chain at least once. Montgomery is another jewel not to be missed.
You might want to check the fire maps in advance to see what impact the big fire had along your intended route. I haven't been back in there since the big blaze.
Later,
Jimbo