You might be pleasantly surprised on both those counts. My Manakamana Special was also posted by Yangdu at 17.5", but it turned out to be a full 18" in length, measured in a straight line from the point to the center of the end of the handle.
Also, the satisal in the picture of my MS looked very dark, almost like horn. That's because the wood had a very fine dark grain that was polished up so it looked almost like marble when I got to see it in person.
These MS knives really are something special.
Looking at that picture I would SWEAR that is horn. If it comes in at exactly 18" that would be nice and I guess I could see it being the same weight as a same length ASTK. Basically the shape of the curved part of the knife is what will differ between them.
Look forward to comparing it side by side with 18" ASTKs and an 18" Siru.