Hey Erich,
well, you know, it's different with the bugs every year, but usually it's for the whole trip. And sure, August is much better than June. Can't compare, cause I didn't paddle in the Barrens, so I think it's "just" Yukon bad.
For wildlife it's really lot's of moose, but usually earlier in the year before hunting time. Very swampy. All in all I've just seen one black bear and one wolf, many beavers, several swans, ducks, other waterfowl and bald eagles. Very good pike fishing in the lakes - some as huge as crocodiles

.
By the way, the Mayo school doesn't do it's annual McQuesten Lake-Beaver-Stewart trip anymore. Some liability issues. Therefore it's a lot more sawing in the small creek. The guy to ask about it in Mayo is Brian McGregor, the former principal.
I have a cabin in Keno City, which is my basecamp for canoeing the Yukon and McQuesten River and Lake are close. This tour is a challenge every time and not just a nice floating down the river. Last time I tried to line up the Beaver to get to the Wind River, but that was to hard for me, so I turned around.
Have fun
Bert