To clean pitting you first clean mechanically.
I use a saddler's needle in a saddler's awl, and do the job under magnification & very bright light, my preferred magnification is a 2.5 - 3 X jeweller's loupe.
If there is the possibility that there is still rust in the pitting, it can be spot cleaned with acid. My preferred acid for this purpose is dilute hydrochloric acid. You can use other acids, but dilute hydrochloric is not severe if you get it on your hands. It will have an etching effect on surrounding areas, but this can be easily polished back. In fact, dilute hydrochloric was used in the beginning days of the damascus revival in western knifemaking, as the preferred blade etch, until people discovered better solutions. Bill Moran used to use dilute hydrochloric exclusively for his etches.
Plumbers use dilute hydrochloric acid to clean copper pipes before soldering, they then may or may not use a flux in the actual soldering process.
However, people who need to clean or stain metal, including kerises do not use plumber's flux to do the job, they use dilute hydrochloric acid. The same stuff that is used by plumbers prior to soldering.
Looking at the pics of this keris, I would give it an overall polish with wet & dry paper, starting with probably 600 grit and working through to 1200 grit. I would then sensitise the blade with juice from fresh limes before immediately going ahead and staining it.
The already dark areas of pitting will be absorbed into the finished stain.
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