Philipp,
My Attachment Relate Info Window Widget uses basically the same triggering as the new PopUpInfo windows in 2.3. I find it strange that you can not have the parcel layer at the bottom of the map using my Attachment Relate Info Window Widget and other layers on top using PopUpInfo windows...
The idea about a button to trigger the widget is not feasible in the design of the my widget, so that's a no go.
Maybe you should show me how you are configuring both mine and the PopUps, by posting your config.xml.