An ill fate"behind the scenes" manual intervention that I performed - appears to have confused Lightroom. The pictures are NOT in library ... yet, it will not allow a "do over" because Lightroom recognizes them as already having been added.
By what particular steps have you concluded that these photos are definitely NOT in your library? (perhaps, remaining in a former location or on a different drive volume, other than the one you are expecting)
In logic there has to be some trace remaining in the Catalog of these pictures as previously imported, otherwise there would be nothing there for an attempt at a second "do over" import to conflict with.
So, to set folder location and file naming aside for the moment, e.g. if you take one of these problem images and find out its date of capture - then make a new Smart Collection which selects for images taken only on that particular date, does that make this image show up?
If so, you can right-click and choose "Go to Folder in Library" to expose what is going on so far as the Catalog. That will work even when the folder this references, is no longer reachable at that same prior physical address (due to some file or folder renaming or reorganisation, and/or a change of connection status or drive-letter / name of some external storage, a change in network storage addressing, or access permissions, or whatever).
In this case the image will still have its thumbnail, and it will still have all its edits History and Collection memberships etc, only it currently lacks the right information to navigate to wherever its physical source file now lives. That is what needs fixing, if you want to keep the work you have already done.
And this is nearly always the best answer, and in fact less work overall, than the "do over" option.
If you don't want to keep that work, a pre-requisite for any new importing of these same images, will be either to first remove all trace of their earlier importation, or else to begin again in another (new) Catalog which lacks all such trace.