Japanese sword blades were often forged with different profiles, different blade thicknesses, and varying amounts of grind.
Wakizashi and tantō were not simply scaled-down katana; they were often forged without ridge (hira-zukuri) or other such forms which were very rare on katana.
The lowest carbon steel is called hocho-tetsu (soft iron), which is used for the shingane (translated as "core-steel") of the blade.
The high carbon steel (tamahagane) and higher carbon pig iron, called nabe-gane, will then be forged in alternating layers, using very intricate methods to form the very hard steel for the edge, called hagane (edge steel), and the springy metal jacket, called kawagane (skin steel).
SFI has a section devoted to the Chinese swords, with some experts in that field, and i would certainly recommend posting the additional pics there.
As for the Japanese tsuka, I think that the fact that they were readily dismounted and maintained would likely be the main reason some are found in such good condition relatively speaking.
You should also better ask in a Chinese sword forum, e.g. Tim, as per my experience with perisan blades too, it's not safe to use the same approach with swords of different cultures.
I would say that there are no homogeneous criteria to be applied because of the sheer number of variables (steel used, cultural approach to maintenance, period of making, site and type of storage, quality of maintenance in the near past, type of wood of the handle, quality of the craftmenship, ). Sorry the photos were not good enough to make a determination on the tang age.
Just think to this : if you want to see a greatly maintained chinese sword of the Tang dinasty you've to watch into a Japanese Jinja. I hope that my friend will be able to send me better ones and I can post those?
I wonder if forum members would agree that Japanese sword tangs show less deterioration due to the ability to remove the tsuka and polish and oil the tang or not more than the quality of the steel or say the climate of Japan?
Four or five people need to constantly work on this process.