For 4 you can implement that with a channel passed along with the message to send a result back. You can then block the sending side all the way to the callsite if you wish.

My feeling is that sans-IO is particularly useful for libraries, although it can be used for applications too. In a library it means you don't force decisions about how I/O happens on your consumer, making it strictly more useful. This is important for Rust because there's already a bunch of ecosystem fragmentation between sync and async IO(not to mention different async runtimes)