relatively unripened peaches that look like they rot from the inside out. What seems to be going on here?

That's all the peaches I can ever find. They don't ripen, they go from hard to rot and they taste like nothing. Just bad peaches.

There are many fungi that can gain entrance during bloom and cause a rot prior to, at or after harvest. I've been told that earwigs get inside them as well. . In recent years I've bought peaches from supermarkets that were advertised at a sale price; they looked beautiful on the outside, only to find they tasted woody, dry, never got juicy - it was then I discerned after seeing a program on television about grades of fruit, the buyers often purchase lesser grades and then sell them to the public. Although most of us hesitate to return these to the stores; it's not a bad idea to do so, in the hopes they will be more accountable to their customers. Also it could be that the fruit was picked too green, and didn't have a chance to ripen at all on the tree.

Parasites, laying eggs in the center, like a wasp and an apple.

Peaches that develop late in the season tend to do that. Let them shrivel up, discard the flesh and you have an almond.

Some young people seem like that too.