dragging photo from photos app to desktop says "missing file"

Are you running any cleaning apps? Those can sometimes delete Photos files in a misguided attempt to remove "duplicates"

Could you have inadvertently turned on referenced files for your library?

Yes that could. You have to give it time to download the original. Try exporting rather than dragging.

What I showed are the Photos Preferences.

Are you running a "managed" or "referenced" library?

Check to make sure it's a "managed" library.

Hmm.... couple more questions.

You double check with the Installer to see if they were still on the Phone?

Do you have a Time Machine backup for the iMac?

If the iMac is and has been backed up using Time Machine, then you may be able to recover the Images from it. see > Recover files using Time Machine and Spotlight on Mac - Apple Support

Click on find original. The photo is probably corrupted.

As a work-around till this is resolved maybe try the export function in the Photos app (under the file menu). It has been bullet proof for me.

Wow, I'd say that the files are somehow getting corrupted in the "save to Photos" step, but don't understand how or why. Are they still visible as a photo on the Photos app? If they are still visible in the Photos app take a screen shot (or window shot) of them and use that for email or other purposes. I know it doesn't help an already saved and deleted old photo, but going forward maybe try saving the original files in a folder on a drive location, then save them in Photos.

You want that checked.

With that checked you have a managed library and Photos stores the original images in your Library (or on the cloud)

Unchecked, you have a "Referenced" library and *you* are responsible for the safe keeping of originals. If you delete them, all you have are the thumbnails in the library. Not only that, but unlike its late smarter brother Aperture, Photos does not handle referenced libraries very well, and you are *strongly* encouraged to use a managed library (i.e., check the box) It is checked by default.

What happens when you click Find Original...?

Are the Installer Phones their personal phones or are they company phones?