I like 'chasm' too. It may be less common, but it's more literary -- that is, I agree with tanja1 that 'gap' isn't as serious-sounding. 'Divide,' also possible. 'Abyss' is also a possibility, but maybe a bit redundant in combination with 'unbridgeable.' 'Gulf' and 'rift' don't work as well for me, though they're possible.
The word is not 'un-' but 'insurmountable,' and it really should be used for an obstacle, something you can think of as having to climb over. It's the difference between down into (or across) and up over.
Neither 'schism' nor 'cleft' works at all for me. A schism is normally a sharp difference in opinion between two groups, and a cleft is a hidden place, a nook or cranny, a narrow space in between.
But I do like 'irreconcilable,' if you want an alternative to 'unbridgeable.'
As for the rest, I might try to give the diction a slightly more consistently literary feel by avoiding ordinary colloquial language like 'lad' and 'take his eyes off it,' and I might replace 'refers to,' which to my ears doesn't work well in this context in English.
Like the eponymous beautiful youth in Greek mythology who fell in love with his own reflection and could not turn his gaze away from his own image until in the end he perished, the 'Narcissus' installation of Bachmann/Banz points to / focuses on the almost unbridgeable chasm between appearance and reality, image and reflection.
I'm not sure I understand myself what the difference between 'Bild' and 'Abbild' is, unless one is meant to be an accurate reflection, a realistic image like a photo, and the other an artificial representation, like a painting or a portrait. I think 'image' in English actually means either or both, but it's been a while since I've read this kind of stuff.