Can't download full resolution photos on iphone

Since my original post, which started this thread, I've discovered and/or realized a few things. My original gripe was that I could no longer view my photos in their original full-resolution form on the camera roll in the Camera app. But they're stored in full-res form in iCloud, and they're viewable through the iPhone's Photos app even if not thru the Camera app.

If I understand correctly, turning on the iCloud Photo Library switch and checking Optimize iPhone Storage will cause the phone to upload all its high-res photos to iCloud (presuming you have enough iCloud storage available), where they will be accessible in full-res form thru the phone's Photos app. Low-res versions will remain available on the phone and viewable thru the Camera app. Have you done the above? You won't have the images in full-res form on the phone, but they'll be accessible through Photos, if your iCloud storage plan has enough capacity. In this case, you needn't fear having the full-res forms deleted from your iPhone.

My experience suggests that full-res images remain on the phone for a period of time before being converted to low-res to free up storage space. This low-res presentation in the Camera app (currently) applies to all the photos on my phone taken 48 days or more ago, but the next photo, taken 38 days ago, and all photos taken after that, are on the phone in full res. Since this only started happening 5 months ago, and the phone is nearly out of storage (I have only the 16-GB version), I'm assuming that at some point where storage shrank below some Apple-established minimum, all photos on the phone were converted to low-res to free up storage, with the originals remaining in iCloud. I also suspect that this conversion is set to take place at some point between 38 and 48 days after each photo was taken, perhaps at 6 weeks. Or maybe it depends on available storage. It will be interesting to see what happens after I upgrade to iPhone X in a few months.

Thanks, but as noted in my OP, I've done this. Also as noted, there isn't the storage available to download the library at full res. But isn't the full-res version of a photo supposed to download automatically when a photo is viewed, or at least selected for editing? Instead, I get an error messag