It's interesting Paul, how flexible the dulcimer can be for a non-chromatic instrument. I listened to that particular clip you found and I think it's in the same key all the way through - but I'd have to listen again to be sure. This arrangement I've been given to work from is a duet - soprano & alto parts. The key change is part of this particular arrangment - I've found a lot of hymns sung by the 2 church choirs I sing in have key changes - I used to think it was how the hymn was originally written, but perhaps it's a case of how this particular version of XYZ hymn has been arranged.
In any case, I only have to play chords for this one. The cellist is there to provide a tenor/bass harmony and so some kind of rhythmic accompaniment is needed, which is the chords on the dulcimer. You know I didn't think of looking at the chord fingerings and how they'd sound on a tuning that's EbBbEb. I will give it a try and compare it with the 1-3-5 tunings. That's a good suggestion thank you. I experimented tuning it all Eb or Bb and every chord was a barre chord! LOL. Barre chords are hard on my fingers, whereas other chord fingering positions are not - go figure.