[David]

Originally Posted by Jean

.The blade has 9 waves and a ganja iras (typical from the area) and pamor Adeg. It probably dates from the 19th century. The top face of the ganja may have been originally covered with gold kinatah.

I am pretty much in agreement with Jean. However i am curious why he suggests that the top of the gonjo may have been originally covered in gold kinatah. I do not see any evidence that kinatah was once in place here and certainly not all nice Palembang keris also had kinatah.
I agree that the tip of the sheath stem seems to be missing something. A flat termination is not generally correct for this type of sheath.
To answer a question that seems to have been overlooked...
"The scabbard could be newer, but has some age, too. The mouthpiece is made of more beautiful wood than the remaining part. Was there a silver mounting to cover this part? Or had it been repaired? Or is that normal?"
I see nothing unusual about the top sheath. I doubt there was ever any silver mounting at the mouth of the sheath. You are correct to recognize this is beautiful wood. I can't tell from you still photos, but i get the feeling that the grain of this wood might display a chatoyancy effect. Such wood is highly prized and there would be no traditional desire to cover up such nice wood grain.
Since keris are not your thing i will explain a couple of comments made here, though you may already understand them.
Gonjo Iras (or ganja as it is sometimes spelled) refers to the fact that this keris was made all in one piece. Keris generally has a separate gonjo, the small asymmetrical cross piece/guard at the base of the blade. It is generally made separately and then fitted to the blade over the pesi (tang). In your blade this is all one piece. You may be wondering what the significance of that is, bit i am afraid i have never heard anyone explain that to my satisfaction. There are more than one theory explaining the spiritual/philosophical significance of these parts of the keris. But i have also seen very knowledgable people suggest that gonjo iras is simply a money saving way to forge a keris.
Pamor Adeg refers to the pattern made by non-ferrous material such as nickel that is folded into the forging. "Adeg" means standing, and as you can see the pattern lines all move in one direction in a parallel fashion.There are numerous variations on this pamor pattern.
I would agree with the 19th century dating, but to answer another of your questions there is really no way to precisely date a keris unless you know the maker. Use of similar styles and materials can span over fairly long periods and the condition of such antique blades is dependent upon how well they were cared for and how often they may have received acid washings and re-staining of the blades in the keris' lifetime. I think it is safe to call your blade 19th century. The hilt could be as old i suppose, but i suspect it might be early 20th century.
The hilt cup and ring (i would not call this a "selut") looks like it is probably silver. If this were mine i would polish that silver up. Silver parts are meant to shine and i would not consider that a destruction of any precious patina. It looks like real stones in the ring, but your photos aren't all that clear. In all probability they might be "yakut" which i believe is cut quartz crystal.