> What is the backing storage for Workers KV?
We're not talking about it because it's not something that's guaranteed, and we may change it in the future. See above :)
> How is "last" determined?
There's no "determining", it's just literally whichever write happens last. This is enough for many use-cases, though obviously not all of them.
> Have you explored other consistency models?
We'd like to offer additional options, to let people build more and more cool stuff. It's going to take time, though. Stay tuned!