@Grimgrak: If your device is able to detect the eraser end (so that input events send by Android carry `eraser? as `tool type?) and you enable `use stylus (hardware detection)? in LectureNotes' input settings (as you should do for an active stylus), the eraser end should work independently of LectureNotes' stylus button settings.
If the stylus is constructed such that the eraser end is actually a stylus button, then you need to proceed as described by GANote3.