Happy ending!
I was curious when you said earlier that STI levers are known for being 'problematic and short lived'. THis does not agree with my experience. There is certainly that opinion among many long distance tourists, who dislike anything that cannot be jury-rigged together on the side of the road, but in reality, these parts are super robust and can last for decades. Others above mentioned spraying to clean out the hardened grease, and you found your problem was the cable... these are the only two failures common to modern brifters.