PSE existing keywords management at import time.

If you are trying to date scanned photos, why are you not simply adjusting the display date in the Organizer?  You can change the date of multiple photos with a single adjustment.

You are absolutely correct.  Elements treats People/Places/Events tags differently than keyword tags.  (I call them Elements system tags, but they don't have an official name AFAIK).  I believe the reason for the differentiation is that People tags are associated with the face recognition data, and Places tags are associated with geotags in the metadata.  (I don't have an explanation for Events except they may be associated with another algorithm for creating them, partly based on file/folder names such as Birthday or Christmas etc.)

I believe you have already figured out that you cannot click and drag a "People" tag from the the Imported Keyword Tags Category to the People hierarchy.  Again, this is because of the association with face recognition within the database file.  If you have an imported keyword name in a file's metadata, you could simply click and drag the file to a new person name created manually in the People hierarchy of the tags panel.  However doing this will generate a generic icon instead of a recognized face in the Named People room of the Organizer.  If that doesn't bother you, you can do the following:

• Select the person's Imported Keyword tag in the Keyword panel. (This will display all of the files of that person.)
• Create a new People tag with that person's name, using the dropdown menu that is available next to the People category:

• Press Ctrl+A to select all of the keyword-filtered files for that person.
• Drag the newly created People tag to one of the selected thumbnails (or vice versa).
• When I do this, I also delete the Imported keyword tag for that person, because it can create confusion when searching.

I believe that when you accept the option to import keyword tags at the time that the file is imported into a new catalog, the Imported Keyword Tag category will contain keyword tags that have been created in other Elements catalogs, even though the metadata has not been written to the file with Ctrl+W.

I totally agree with you that there should be an option to treat People tags as keyword tags (and vice versa), with the understanding that there are some limitations to doing this, as noted above.