Stereo cartrdge now playing mono!!

The cartridge was replaced with a new one. Very happy!

this is kinda perplexing since I cannot think of a failure mode that would turn a stereo cartridge into a mono one.  

The fact that you reversed the phase of the right channel (swapping red and green) suggests to me that you were mischaracterizing the sound of one channel with inverted phase as mono but this doesn't explain why the hum would suddenly show up.

glad to hear the new cart is singing.... Enjoy!

dave

You will likely find your answer here https://www.vandenhul.com/wp-content/uploads/2019/09/Phono_FAQ.pdf

As A.J. explains, many times it is due to the very fine wire from the magnet assembly/transducer to the pins sustaining fatigue-failure. The very fine wire is often not long enough and becomes brittle with use. When it fails there is no repair. The manufacturer just replaces it. At a price. 

The hum could be coming from the tonearm too.