Imported photos have disappeared from Photos library

Is your external drive formatted in either APFS format or Mac OS Extended (Journaled) format? Other formats cause real problems.

Your current Library is less than 4 months old? When you say "import," what do you mean? Did you use File>Import to get pictures from an SD card, or what? Do you still have the originals? Were the edits done in Photos in this Library? "Imports" is not an entirely reliable view-- have you checked Recents? Have you checked "Recently Deleted?"

You saw these pictures, edited, in Photos-- and now you don't? One of the first things I think of with lost pictures is that you might have inadvertently saved them in a different Library. I sometimes get confused about which Library I'm in at the moment. Apparently you have lots. You could close Photos and option-click on the Photos icon to get a list-- maybe you accidentally made a misnamed one.

Do you have  PowerPhotos ($30)? This is very handy for dealing with multiple Libraries. You can do a search for particular pictures through all the Libraries.

These all seem like trivial things, but they are the first things I'd do to find missing pictures-- in part because, if they don't work, the pictures may be permanently lost. It's like looking under the light post to find your keys even though you lost them in the dark parking lot.