In demo 4, it is shown how an ICustomAppServices interface is created (that the plugin can use to call host service) and a custom ICustomPluginInfo interface that the host can use to call a plugin service.
With these two custom interfaces, you can add bidirectional customization for the plugin framework.
A plugin can hold multiple forms. Not sure how / when you want to invoke these forms from the host, but there shouldn't be a limitation to add multiple forms.
I missed that sample! That should make it fairly easy to populate a TAdvToolBarPager menu from the plug-in.