Can your buddy lower a gopro (waterproof) on a pole and look around under the ice in various places?
Future state, they make wifi enabled remote cameras that are waterproof. Would be cool to keep it underwater. Of course once battery is dead you can't easily get to it at your location and elevation.

I too am stumped at why your CFM, and HP can't keep a hole open all winter long. It makes you wonder if there is air leak between the pump and the diffuser so that the boil isn't as powerful as it could be. You don't have a good sense of the boil plume before it iced over?

Going by Joey's conditions perhaps a plume of running water mixed with air would work. Maybe use your stored electricity to run a water jet plus an air bubble plume? of course perhaps in WI the springs are ground water temp (maybe 50-52 degrees) and you would be circulating pond water (37 degrees or colder). Your cabin have a well or is that out of the question at your altitude and with all that rock under you?