It's an interesting situation, but assigning liability to coinbase for this seems like it'd be a problem long term. Unlike traditional securities, there's nothing stopping me from going out and making my own fork tomorrow - and then would all exchanges be required to add support for that fork, too? Where do you draw the line?
It's a good way of looking at it. Another perspective, which makes this issue so complicated, is that we could compare this to: a company on the stock exchange splitting into two companies, investors should be getting shares in both companies, but the brokerage decides they are not going to recognize the second company and thus the investors don't get their shares in the second company.
No, that is not what's happening here: Bitcoin is a consensus network where consensus is achieved essentially by mining sequence of blocks, which requires a huge amount of computational power. Each block have a relation with the previous block. Sometimes two or more blocks are mined almost at the same time (creating a fork), miners then choose one and keep mining as fast as they can. At some point one fork will be longer than the other/s, which is when consensus happen: miners abandon the shortest forks and keep mining on the largest one.