"Do you have a hypothesis for how a piece aluminum from an aircraft shot down 70 miles south of Baker ended up on Gardner island?"
No, only guesses. One guess is that the piece found on Niku is NOT from one of these three Japanese aircraft. It makes more sense to me that the piece came from some aircraft that made its way to Niku at some time, and broke apart there. Another guess is that it was salvaged from a wreck on a different island and was brought to Niku by islanders. The presence of Japanese aircraft getting shot down in the general area was previously considered unlikely, yet here are 3 examples. I think this is enough evidence to consider Japanese aircraft wreckage, either having crashed on Niku, or imported from islands nearer than we thought possible.