2. Each app defines what intents it can handle, so I don't think curation is necessary. The app select popup window is already curating that for you?
3. This context knowledge is provided inside every intent. Not all apps choose to use it, though. In some cases I'd guess the sandbox would get in the way as well. I'm not sure how much practical value this would have anyway? Can you explain it a bit more?
4. There is nothing stopping Apple from adopting Android intents, but obviously they won't. This is a 'pie in the sky' suggestion that won't happen for the same reason there is more than one flavour of Linux. It's like asking why you can't pipe results from your unix system to a Windows system - it just won't happen for political reasons.