> if you wanted to modify your email client, or at least the one above (Mail.app, the default client for Mac), there is no clean way to extend it. There are no plugins.

Mail.app supports plugins.

> Why can't I have a file in two places at once on my filesystem?

So…a hardlink?

> Why don't my native apps do that?

Dynamic text lets you do this, but it's mobile-only currently.

> have started deprecating the Applescript bindings which make it work underneath

Since when?