If you ask whether Heungseon Daewongun or Empress Myeongseong caused the fall of Joseon, it is Heungseon, without a doubt.

It is a common tactic for a monarch to take all the blame for the fall of a nation,
but since records from the late Joseon era remain, we can perform a qualitative analysis.
To determine responsibility, let's look at some "possible worlds."
Assuming all other variables remain the same:
If King Sejong the Great had possessed Gojong to rule, could Joseon have survived? Impossible.
If King Sejong the Great had possessed Empress Myeongseong to rule, could Joseon have survived? Impossible.
If King Sejong the Great had possessed Heungseon to rule, could Joseon have survived? ...
What if Heungseon had not committed blunders like the Gyeongbokgung Palace reconstruction, the persecution of Christians, the issuance of the Dangbaekjeon currency, or the insulting letter to the Emperor of Japan?
Would it have been... possible?
The answer is clear.