1. Are there any plausible explanations as to how the bones ended up on the island, if they are not AE's (or FN's) bones?
As Pat Fontaine said, not that we've been able to find. There's nobody else missing in the region. Norwich City had a
crew of 35 men, all of whom went into the water in the storm. Twenty-four made it to shore alive. Of the 11 who didn't, 3 bodies washed up and were buried by the survivors. The other 8 are unaccounted for and were presumed drowned or taken by sharks.
The castaway whose remains were discovered in 1940 were found with part of a woman's shoe. Forensic analysis of the skull suggests the castaway was female. Several of the artifacts TIGHAR has found on the site which appear to be associated with the castaway are female gender-specific (mirror and make-up from a compact, hand lotion bottle, freckle cream ointment pot). There were no women aboard Norwich City.
2. Do we know that the bones were disposed of/buried and are they lost for ever or are there efforts to locate them?
There is no record of the bones being discarded but exhaustive effort to find them (three expeditions to Fiji) have failed to find any trace of them. Hope springs eternal but when you consider that the bones were ultimately judged to be of no significance, it seems most likely to me that they laid around until they eventually got in somebody's way and simply got pitched. Why would anyone hang on to them?