Can't speak for the Horse lake area but we were in the Stuart Lake/Boulder Bay area last week and the mosquitoes on the portages were the worst I've seen and 10 years. Lots of rain in June and July got the hatch going good. The other bugs were not bad. Bring your deet, wear long sleeves and a bug net comes in handy. It's 90% attitude.
Nothin' like portaging with 30,000 of your closest mosquitoes friends....
The water levels in our area were pretty good for August. Don't expect to be floating thru any portages but then I would think you will not be dragging bottom too much. Don't expect clean paddling on the Horse River in August. You will need to pick your way.
I think you will be OK.
I can only hope that when I die, my wife does not sell my fishing equipment for what I told her I paid for it