Hello Pilot DAR,
As someone that has no knowledge whatsoever of the Twin Otter, from what you described, there was one detail that I found a bit uncommon when you mentioned 'belly tanks'.
The aircraft has its Fuel Tanks not in the wing itself, but instead this are located in the 'belly', or it is a combination of both locations ? or I misunderstood something ?
Tanks in the belly would allow a more simple transfer of fuel of the Cabin Fuel. I'm guessing it will be done by gravity IF, as you mention, venting is working properly.
Transfer of fuel from Cabin to a Wing tank would imply some sort of pumping to get it done.
As I mentioned earlier no knowledge of the system arrangement, but the Cabin Fuel should be designed in a way that wouldn't require that long to make sure that it is working properly.
From what you describe, it seems to be a very tricky system, that can end up in a very bad outcome.