inspector13: "From the topo map and existing trails (roads?) it doesn’t look too bad to go from Mountain to Arrow Lake through West Wabindon and Little Whitefish Lake. But I am just speculating."
To further speculate, there's a secondary road that heads straight north from Mountain Lake towards West Wabindon for about half that distance, perhaps this is follows part of the old portage trail. West Wabindon to Little Whitefish may be a navigable creek.
To my eyes, looking at the topo (and admittedly not being on the ground), Mountain to West Wabindon, then east to Wabindon and then over the narrow strip of land to Arrow is also a possible route.
Alternatively, there's that big swampy area with a creek between Wabindon and Mountain. If that was followed east, it would connect to the Arrow river a few miles downstream from Arrow lake. Would that creek be navigable? Maybe, especially with a lightly loaded small solo canoe. Which, now that I think about it, may be the reason for the Voyageur's detour - perhaps whatever route it is, a small solo canoe could be paddled for most of it, but the larger Canot du Nord at 20 '- 25' may not be able to, thus encouraging the Voyageurs to find a new route.