Retaining existing keywords when copying all metadata from one picture to anothe

[StarGeek]

I would need to use some other tool besides Explorer that allows me to see dozens of photos visually then select multiple photos and make changes to multi-value fields (adding/changing/removing tags from all fields).  I'll hunt for a tool.

You should look into a Digital Asset Management (DAM) program.  These programs will allow you to easily see, sort, and rewrite metadata in batch.  Some examples are DigiKam (free), Lightroom, or IMatch (both paid).  They will also write data into locations that follow the standards and will be compatible with programs you might transfer you files to in the future.

You mention using the XP* tags (XPKeywords, XPAuthor, etc).  You should take note that these are pretty much Windows only tags.  Almost no program other than Windows will read these and you would have to re-edit your files if you move to a more powerful workflow in the future.