Your folder-level script is correct. The reason it didn't give you anything when you tried to get the user JavaScript folder is because you need to create it manually, on Windows at least. More info: The Acrobat Ninja: Acrobat 10.1.1 JavaScript changes
They are not trying to make it easier on developers, but they have good reasons that are security related. If a script in a document could execute any menu item without restriction, it could at least be an annoyance and a big security hole at most. Security of their applications will always win out over your convenience.
A user can choose to allow this JavaScript method, either by installing a folder-level script like you have, by disabling the restriction in their preferences, by modifying a white list in the Windows registry (I think), or by trusting your digital certificate to allow for privileged JavaScript to be executed without restriction. So whichever method is used to bypass this restriction, a user has to make a conscious decision to allow it.
This latter method gives you a way to include the app.execMenuItem alone in a button action. You will have to digitally certify the document and convince your users to add your certificate to their list of trusted certificates and trust it to allow privileged JavaScript, at least.