I did get it a bit wrong:

Because I assumed that ViewNX2 adds tags into the IPTC area of the image, and that both Gallery and Windows Explorer can add too or edit those same VNX2 generated tags, I assumed that the area of the raw file that both Explorer and Gallery write too was the same IPTC area.

I shood'a checked(I have the tools) .. but anyhow.

using Exiftool, with the ExiftoolGUI front end for it(I'm not a fan of commandline! )

Tagged via Gallery showing the gallery made tag info in the XMP area.
catalog-tag_via_gallery_viewed_exiftoolgui.jpg

A screen shot of exiftool showing the exif status. Note the selected tab on the RHS. I initially checked the IPTC area which was blank .. panicked a bit and then checked other areas.
the tag data is written to the XMP area(which actually makes sense).

Tagged via Windows file Explorer showing the same tag style.

catalog-tag_via_win_explorer_viewed_exiftoolgui.jpg

The anomaly at the moment(for me) is why is it listed in the subject heading in the XMP area?

Like I said tho .. ViewNX2 can see all or most tags, and VNX2 is very fussy with the file types you point it too, chucks a wobbly if the file is in any way not standard Nikon protocol.

catalog-tag_via_win_explorer_viewed_VNX2.jpg

So while it's an issue I want to look into just in case there is future issues for compatibility or something .. in effect it's not an issue.

I did find an interesting thing about Windows Explorer too tho. Initially when I added the first set of info(the tag_test_info_etc) all it did was allow me to add it.
As I added more info into other various areas, if the tag info was already recognised in its database, it comes up as a drop down list to select which ever you prefer(via tick marks).
So it's not as restrictive and rigid as I initially thought, but there are some strange location anomalies to be a bit wary of.

catalog-tag_more_via_win_explorer_viewed_exiftoolgui.jpg
eg. I also added more info(after the above screen shot from ExiftoolGUI) and added a comment and subject and title.
While the info added seems to make sense here(look at the XMP location names where the metadata is located .. under the XMP tab, the title is fine and labelled 'file explorer test title' the problem is that under the subject heading in the XMP tab, the same metadata is listed, and not the subject I added labelled 'file explorer test subject'.
The added comment 'this is not as bad as I thought' doesn't appear anywhere else other than Windows Explorer under the comments tab. Neither Gallery or VNX2 see this info at all.

While it's easy to get caught up in some semantics over who wrote what where and why can't the others see it .. in the long run I don't think it's important enough to worry about.
From a what if the worst happens point of view .. I think that the important part in all this .. at least for me .. is that the metadata is where it should be first and foremost .. in the file it pertains too.
From there a catalog can be made from which we then find our stuff.