Q: Aperture reports "unavailable original images" although no "offline images". What to do?

"Unavailable original images cannot be consolidated." I went in and searched for offline images, and there are no offline images.

If it should happen again. search also for the "Filestatus is missing", not only "Offline"

Planning to change some image dates and set some locations within Aperture before migrating to Photos.

That is a good idea.

The new Photos.app in El Capitan can assign locations, but the maps in the Info panel are very crude. It is easier to assign the locations in Aperture. And Photos cannot write the metadata to the originals. If you want to write metadata to the originals, do it in Aperture.

Also any batch changes you want to do. You may want to rename keywords, if you are using hierarchical keywords. The keywords in Photos are flat.

See:  How Photos handles content and metadata from iPhoto and Aperture - Apple Support