[SalesForce] exclude button from page layout in package doesn’t seem to work

You can reference a visualforce page which resides outside of your managed package in controller which resides inside the package, by not using Page.VisualforcePageName, and instead use new PageReference('/apex/VisualforcePageName'); because say if you wanted to reference a Page inside your managed package, you would add the namespace: new PageReference('/apex/packageNameSpace__VisualforcePageName');

You won't get a compile error in your managed package when you do it this way, because the existence of your page is evaluated at runtime, and would display an error when the page wouldn't exist.

Take a look at a question I asked, which might be helpful to you:

Make visualforce page (pdf) available outside managed package

Yes we've done this and been successful.

However your correct to be cautious about it though. We've had things go wrong under the hood in the past (typically with more complex dependencies than your describing here though). So long as you're not under major pressure to release Salesforce support have in our experience been able to pick through things for you and sort things out, just be sure not go messing around further if things appear odd.

Uninstalling from Packaging Org. So long as you no longer have any references to it, it will uninstall as normal, even from a packaging org. If however you've made references to it via fields lookups for example, that you cannot remove or edit out from your package, because those have been released you will not be able to break the dependency. This doesn't mean you cannot install the unmanaged version, as this will be in the your packaging org namespace. Its just that it would look a little odd to admins in your subscriber orgs to see the same classes, objects etc from the Action Plan namespace and your package namespace, other than this urglyness that may be tolerable.

Uninstalling from Subscriber Org. This again will be possible once you the subscriber org has upgraded to your new dependency free version of your main package.