Just curious;
aren't there services for staining iron at all? Some contracters who do protectionwork for your fence or interior? Probably they are using similar substances. Maybe iron staining is used in some other proffessions as well? Who knows if they can stain your fence, they might be able to do your Keris as well
(I havn't tried since I have found a pop & mom place next door).
For the sake of keeping your item in a good condition I would recommend to have it stained. (Keris) Oil can however already have a nice effect, since it normally brings a bit of extra contrast. But he, its your spear, please do with it what you want.
Regarding its origin I hardly dare to comment since Alan has given an opinion already
From the pictures included my uneducated guess it is central java made around 18th century. In case the ferule is not seperately forged, place and time might differ. (madura)
The little holes might have been drilled in later times for decorational purposes, I dont think they are part of the original design.