I was totally unfamiliar with harp guitar but I am sad that I missed such a special gathering!
If you haven't heard them yet, you're in for a wild ride: https://www.youtube.com/@AnginePoitrine
https://collectionsdumusee.philharmoniedeparis.fr/search.asp...
I see your added frets, and raise "add frets on the fly using Lego":
Really? I highly recommend Flying Microtonal Banana.
Really? Most people's first intro to microtonal guitar is recent and brought to them by Angine de Poitrine.
You only have to put your finger somewhere in between two frets to get a particular note, so you can play more than the tiny handful of chords that can be played on something like a violin.
There are non-fretted guitars but they are rare, and I think early banjos were not fretted.
What do you think double-stops are?
Of course in most music contexts there is more than one instrument playing and so the others can fill in the missing note making a full chord. Only a "one-man-band" really feels the loss.
That's E, G♯, B, D, F
https://www.youtube.com/watch?v=lz3WR-F_pnM
If not, stop and watch it. But make a cup of tea to enjoy with it first, and maybe put a cloth over your keyboard for when you inevitably splutter tea all over it.
in college a good friend was a very good Oud player (plucked, some number of strings.... more than 8? I forget), and it definitely sounds different from guitar due to less sustain. Clearer notes and chords, but they don't linger as long or loud.
Edit: apparently it's 11 strings? ok.
https://en.wikipedia.org/wiki/Category:Guitar_family_instrum...
https://en.wikipedia.org/wiki/History_of_lute-family_instrum...
They are as old as we can trace it back, vihuela/viola de mano were multi-course, as was the baroque guitar which developed from them. The romantic guitar brought us single string courses and in the guitar family double courses died out other than in a few nooks and crannies. I don't think this has any effect on what TFA proposes and the author is well aware of this bit of information, but it has been years since I read it and maybe I am forgetting something?
As a matter of fact, the "tenor guitar" was made for people who knew how to play the very popular four string banjo, but wanted to play a guitar without learning new chords.
Instruments were in a state of change, and the "harp" nomenclature (and styling) for a 12-string guitar was more a marketing flourish than a family lineage.
Would that be a ukulele? Or is that something different yet?
If you haven't heard (or seen) Michael Hedges play a harp guitar, there's some background and links to videos here:
https://michaelhedges.com/michael-hedges-the-harp-guitar/