Comment by fipar

I don't know about the first two because those aren't a use case for me, but for 3, you want helm-org-rifle-agenda-files, and for 4, I don't know about something org-specific, but you can use emacs' search commands after narrowing the focus to the current subtree (by default bound to C-x n s, I use this so often that I have it bound to H-f, and I revert focus with s-f, so that I am not distracted while working on an entry).

The first two are interesting use cases though, I'll reply again if I find something that works.