Existing Configuration

Is there a repo for user defined configurations for MP£Tag?

I think the list at https://help.mp3tag.de/main_tags.html is not exhaustive, and I'd like to understand exactly what information can be held in an .mp3 file. I'd like to show ALL tags that are available in an .mp3 file and thus visible in MP3Tag.

Are certain tags strictly numerical only? I see MP3Tag has a check-box in the customize columns dialogue - does this indicate that numerical only input is OPTIONAL? OR are some tags strictly numerical only?

This option refers to the sorting and display of that column. A numeric value is displayed right-aligned. It is then sorted by numeric value and not alphabetically.
You can input more or less any string in any field.
The field TRACK is numeric only in MP4 tags and numeric plus a slash in ID3V2.3 tags.
Some fields require a certain format like RELEASETIME, UNSYNCEDLYRICS, INVOLVEDPEOPLE - but this is also described in the help.
To see all tag fields in an mp3 file, use the extended tags dialogue: Alt-T.
The dialogue for extended tags also shows user-defined tag fields.
MP3tag does not show PRIV tag fields.

My apologies - I meat available tags - not already populated tags in a specific .mp3 file.

You know where to find a list of tag fields that are supported by MP3tag.

If you want to dig through the standard, see
http://id3.org/d3v2.3.0
and scroll down to
"4. Declared ID3v2 frames"
where you will find a list of fields defined by the standard.
Again: this does not mean that the list of displayed tag fields is complete as the standard also allows user-defined fields.