Welllllllll.....I'm a young 58...probably closer to your Dad's age.

Black Bear Portage used to be owned by the Seefield family. I had a huge crush on their youngest daughter (Iris) back in the day. I'm not sure if Margot is one of their daughter's too?? I know Iris had an older sister who lived in the area.
The river NEVER freezes under the CN bridge...and I've been up there when it was -50....but it's a dry cold??

That makes a lot of Roughrock problematic with the current cutting right through the heart of the lake. I have ridden the trail on Big Sand and up past Sand Lake Outpost. And it does utilize the old logging road that runs the ridge between West Bay (Big Sand) and Black Bear Portage. From there the trail runs due North and doesn't really get anywhere near the Whitedog Dam. Traffic IS light and grooming is waaaaaaaaaay different than what we are used to in WI or the U.P. Once every week or two is the grooming schedule for that trail. And where a trail isn't running an old logging road, they can be somewhat difficult to groom and maintain....you've seen the terrain. LOTS of bedrock.

Should you ever decide to go sledding up that way, I would be more than happy to help you where I could.
BTW, I don't know if you trailer your own boat there during the summer, or not....but you could save quite a few rock chips on your boat and tow vehicle by launching someplace other than Roughrock...those last few miles can be pretty brutal.