My theory is that this was no "accident" and that it was much more than just a tunnel collapse. It's just a theory on my part but I think that our military carried out a devastating attack on the nuclear test site to show Kim what we were capable of.
I think the N. Korean's squeezed off one last missile test after that but there has been nothing since.
Don't forget that Trump made an Asian trip in early November. So the October timeline makes sense - we wanted to remove any imminent threat while POTUS was over there.
That would explain better Trump's provocative tweets to N. Korea in November and January. Knowing that the N. Koreans posed no further immediate nuclear threat, Trump was not risking much with those tweets and if you read between the lines, he was basically challenging them to come to the table and work things out before things got to the next level.
I agree with your theory... good analysis. What are the chances that this tunnel collapsed at that particular time?