-What remains of these buildings at the site? Were they ever intact since TIGHAR has been visiting Niku, or were they already in a derelect condition by that time?
They were apparently secured and left intact for a while but at some point they were disassembled. I'm aware of no report that they were bulldozed over but there is no sign of them now. I think the story Marty is thinking of about the bulldozer being driven into the sea is an incident where the dozer fell into a large groove while pushing a landing craft off the reef. It was subsequently recovered.
-Did the Coast Guard ever return to retrieve anything, or basically abandon the remaining material?
There appears to have been an initial assumption that the CG would return to either salvage the equipment or re-activate the station, but we have found no record of that happening.
-Is there any evidence that the colonists made use of the materials left by the Coast Guard at the site?
Plenty. The abandoned village is dirty with stuff that was obviously looted ...er ... liberated from the station. At first a colonist was hired to live at the Loran site as security guard but it appears that, at some point, it became apparent that the americans weren't coming back and the colonists said, "What the hell." and made off with anything that looked useful.
-How did the Coast Guard secure drinking water for their station? Distillation or shipped in?
Big cistern. Still there.