Answer
The custom Logical Unit itself needs to be explicitly added as an entity inside the Custom Projection — an auto-generated projection doesn't necessarily carry every related custom LU into itself automatically, especially if the page references more than one custom LU. Once the correct entities are added to the projection and the projection configuration is published, go to the Aurena Page's Page Configurations, select the entity you need first and save — only after that selection is saved does the corresponding 'Data Source Entity Set' option become available to pick from, since it's populated based on whichever entity was already selected. This selection-then-save-then-datasource-becomes-available sequence, done for each entity the page needs, is what resolves both the blank 'View Page' content and the missing Data Source field.