It now seems to work for me! I open up the photo in Photos, click Edit, then click the circle with the three dots, and edit with either the specific Affinity controls or even Edit in Affinity Photo . I make my edits and then Save. My edits are visible in Photos,. I tested whether the edits persist in the file by dragging the edited photo to my desktop and opened it--and the edits are still there. And the edited photo is still in the same position in Photos where it had been, among the others taken at the same date, so my organization is not changed, which is important to me. (I definitely feel I tried this before and it didn't work, so I don't get why it seems to work now.)
What does NOT work is the procedure I used to follow in Photoshop CS5, in which I'd select the photo in Photos and chose "Edit with" and then select Photoshop and open PS. Doing that with Affinity Photo, opens Affinity Photo and you can make all the edits you want, but then they seem lost once you close the document.
Another odd thing is that if I do successfully save edits as described in the first paragraph, later opening them in Affinity Photo as described in the second paragraph above, the photo doesn't consistently show the edits that were made. Sometimes it does sometimes it doesn't. (Perhaps they don't show because the Photos editing feature is nondestructive, saving the original and allowing you to revert to it--and AF then pulls from the original not the edited version.) This doesn't seem like a big problem--just be consistent and use the method in the first paragraph. However, it's still confusing to me as sometimes opening a photo as described in the second para, shows the edits and sometimes it doesn't. Sometimes only some of the edits show. Go figure.).
