I’ve decided to file the issues on the photo overview page as a bug report. Some things there just don’t add up:
By “curating” my images of eggs into the main set of photos for this species, these now do show on the main page, but on the photo page I still cannot select “Life Stage” eggs.
Also, as melodi pointed out, it’s maybe a bit weird that even Quality grade “Any” keeps all captive photos from showing. Had this not been the case the discussion would have been unnecessary.
P.S. Bug report filed:
https://forum.inaturalist.org/t/issues-with-selectors-on-species-photo-page/19792