Hi Michael,
I strongly doubt the whole Steller seacow story:
A: Its an artic living animal and of course its p[ossible that someone exported the bones to Borneo but actually I don't see any reason for that.
It should make the handle extra expensive while dearantler has almost the same effect.
B: the structure /grain of your examples are following the fork shape while a handle from seacow most likely should have all the grain in one line.
C: dearantler can be enourmous various in quality and color and it looks to me that ( the Kenya like) is carved quite near the spongeous centre.
If you come closer to that centre you have indeed more "bonelike" structure on the surface.
D: What I doubt if a animistic Dayak would easily accept to take a "dead ?'' bone from an animal he doesn't know and carve from that a handle that he carry with him the whole day.
Antler has a special reason as its one of the "fastgrowers" ( like hair, nails,tusks, horn) and so "loaded" with "living power",sometimes also special kinds of wood ( specially shaped roots etc).
E: About that human bone handles, maybe they are human and seacow as well, carved from the bone from an anourmous fat lady.....
Arjan