Maybe you misunderstood. The images in Post #55 are of the inside of your tennis racket zapper. If you can expose the circuit board maybe you can safely measure the high voltage.
The voltage divider I am suggesting you build would be one like you would build to reduce the signal voltage to an Arduino for safe measurement except you would have to use components rated for the very high voltage you are dealing with.
There is no need to open up your DMM as the voltage divider would just be lying on your test bench and could easily be touched with your meter probes.
However, the best thing to do is just find another puzzle to work on.