Windows API is for how data is read from the disk or written to it, you can't just go and make a plugin as I said to allow copying differently and make it part of explorer, you'd have to use hooks and register it and whatnot which in itself is a hack.
They do. It's called the Windows API. No file copy utilities have had their own disk access systems since DOS.
I haven't tested it with network connectivity so what happens if you're on an network, start copying from a server, pause and disconnect from the network then reconnect later, can it still be resumed or not?
You can pause/resume with windows 8's file copy...