Hmmm. Hendrix example was just showing how he used a normal guitar, but upside down so to speak. Since hes fingering fretboard w/ right hand, the highest pitched strings are closer to ceiling, and lowest pitched strings towards the floor. OTOH, there are guitars that have the strings "reversed" so that if he looking into a mirror, that image would like he a "right hander" playin a normal instrument? He doesn't play, however, the "specially designed left handed guitar".
Am I making sense? lol
There are only 12 major triads. How did come up with 15? Same for the minor triads. Well, if you include inversions, I guess that is technically 36 spellings, but 1/3 of those are not typically used as a "major chord", but what they call 6-4 chords, more for cadences (endings of phrases, movements, etc). Here's a quick google result when pluggin in 6-4 cadence:
http://musictheoryminute.com/Cadential64.htm
The chromatic scale is the easiest to play, arguably. Definitely takes the least amount of brain work. Because every note on the instrument is part of that scale.