The guiderail placement is certainly unusual, usually I see it placed behind the sidewalk at least for new installations. Since the rail preceded the walk at that spot, I guess DOT didn't want to redesign/recalculate a new location, even after it looks like it was replaced in-kind at the same location. I don't think that placement is better for pedestrians since the guiderail will give a little bit in a vehicular crash, but it does keep them on the sidewalk rather than walking right onto the narrow road at the overpass.
As to the AC interchange, Great Island there was cleared in that area in the early 1960s (Historic Aerials) with the interchange constructed in 1970 (HA and bridge marking). All I could find with regards to a specific intent was that a local developer acquired the island in the 1970s from Vornado to build a casino there. Obviously that plan fell through. It may have also been a spot for a relocated Bader Field before the city got it for the school and public works facility.