Well that's what they claim. It appears to have no soundbox so can only assume it's electrically amplified.
No idea. Couldn't find a lot of info about it.
Maybe it was cello

I seem to remember them saying 5 octaves...?
...just found this:
http://heroeswiki.com/Shenkar"A child prodigy, born into a renowned family of musicians, Shenkar studied vocal from the age of two, violin from the age of five, and performed his first concert at the age of seven. His 1980 release, Who's To Know, and Phil Collins's solo debut, Face Value, introduced the unique sound of Shenkar's own invention, the 10 string stereophonic Double Violin, to listeners around the world. The Double Violin, designed by Shenkar and built by Ken Parker, covers the entire range of the orchestra's double bass, cello, viola and violin. The combination of Shenkar's mesmerizing vocals, double violin and also as a composer has brought him worldwide acclaim as a musical innovator."