I was stationed on Adak Island mid 70's. Brother sent me copy of Thousand Mile War. No mention of fighting on Adak. so How did Japanese remains come to be buried in Adak Cemetary? from wikipedia "In 1953, remains of 236 Japanese dead who had been buried in Adak Cemetery were reburied in Japan's Chidorigafuchi National Cemetery."
There was a US Airfield there so they may have been casualty or prisoner evacuations who subsequently died being treated or held at Kiska.
I think they just took all the bodies off, American and Japanese. Attu is just soft tundra (like walking on a waterbed in most places), so they couldn't have used any sort of equipment to bury the thousands of bodies. They just bagged then and dragged them down to the ocean to be loaded onto landing craft. There were only a dozen or so prisoners taken so no, these weren't prisoners who died in captivity. I know that a bunch of Japanese dead ended up buried on the mainland near Anchorage, though I don't recall the details. These bodies were also repatriated to Japan at some point. I spent a year on Attu in the mid 90's. It's an unpleasant place.
The Japanese bodies on mainland Alaska are (were?) at Fort Richardson National Cemetery, Anchorage area. There is a very nice monument to them. Interestingly, there are also some British graves in this Cemetery.