No expert on materials and their manufacturing but I'll toss in my opinion too.

I can't remember the last time I saw a broken inside plate, likely I have but so infrequently they didn't even register in my long term memory (and odd stuff usually does peak my interest, like this post). My guess is that the connecting link had nothing to do with this. So too with any cutting of the chain by whoever did it at this link. For the reasons that the pins are not a press fit within the inner plate and that this crack isn't even at the pin hole anyways. The pin interference fit (with the outer plate) can cause radial cracks at the hole, and there's a history of chains being improperly heat treated during manufacturing, helping to cause these cracks. But without any stress from the pin acting on the inner plates I don't see how the pin can cause this.

What I do suspect is the manufacturing of this chain/inner plate, was "off" and it finally gave out. After what many might consider enough miles to consider the chain having "earned" its keep. I doubt that without a serious and expensive metallurgical assessment we won't really know what caused this. But thankfully the fix/chains are not expensive, even when the cassette needs replacement too.

My suggestion is to replace the chain (and cassette if that's worn beyond problem free use too) and enjoy the ride. Andy