Spiral tuning has a lot of chords and is good and intuitive for melody playing. A disadvantage of the Spiral is that all chords only go upwards from their root, and thus get rather boring compared to the larger chords of Richter based tunings. Also, many chord progressions force you to make large jumps that make you sound like a beginner at a piano. One, or maybe two, invertable holes could fix both these issues!
Take a look at this combination of Spiral and PC with notes in hole 3 interchangeable:
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⇅ (⇅?) Blow: D F# A C# E G A C E G Draw: E G B D F# A B D F# A
IF it is not much harder to make two holes than one, you could include hole 7 as well to gain an Em and half a G, and basically make the draw notes one long D chord. That's just *if* though. Just the one hole would remove many of the disadvantages of the Spiral.
The range of this harp would let it play most fiddle tunes, with the typical normal range of a trad fiddler being a little over two octaves from a G to a B. The only major drawback I can see that's left from the Spiral is the lack of a proper bend in hole 4.