Regarding an unwound bass string - Noter drone players may use an unwound bass string of 18-20 gauge plain steel (guitar, banjo or piano wire). This string is never fretted but used as a drone. And used this way a plain bass string produces the high silvery timbre many noter drone players want from certain instruments in their collections. I use plain gauge bass strings on some of my noter drone specific instruments (usually made from piano wire). If you do intend to fret a thick plain gauge string at a relatively low tension then the intonation will be poor to awful
Thick plain gauge strings also have reduced sustain compared to their wound counterparts - so if you intend to fret the string up the fretboard you'll tend to just get an out of tune 'thud'.
Regarding back fretting noise: A slightly higher action at the nut or using a dampening finger kills this.
Regarding an octave lower bass - You can do this but will need quite a thick string (around 0.054) and higher action, plus wide nut and bridge slots. However, the dulcimer body itself is not capable of producing the fundamental low frequency D2 (or D3 for that matter!!!!) So the string will sound 'boxy' rather than rich.
Basically, the mountain dulcimer, at the body size, shape, VSL and pitch we use today was never designed for playing chords in DAd or CGc. Our modern instruments are based on the size and shape of older instruments that were played in noter drone style with a different set-up, tunings and strings (for which the box size and shape sonically work well). So contemporary chord melody playing is always going to be a compromise. As you say, it is not expensive to play around with string gauges and set-ups so it is well worth you experimenting to find out the set-up that will suit your style of playing the best.