Multiple notes for an item isn't currently possible, but I'm reasonably sure that we have a feature request open on that; if you email our
support ninjas, they can attach your email to that item so the rest of the team knows you'd like to see this added.
Regarding the "collapse all items" - there isn't a built in feature that does this, but it's not far from what some of the scripts in the
Useful Scripts package do. Since AppleScripts can be added to the OmniOutliner toolbars, it wouldn't be too much work to give yourself a button that does this. (Installing AppleScripts as toolbar buttons is covered in the "About" document that comes with the package.)