SO THE MYSTERY DEEPENS..
The educator I met with yesterday at the museum suggested I contact the newspaper in Oslo (Aftenposten) to see if I could search their archives. Interestingly, they had a chat button on their web site and I chatted with Marcus this morning who found this...
Thor's death notice in the Oslo paper - but how he died is written only that he died "plutselig" or "suddenly" - again no definitive answer as to his cause of death.