During a kantei, the nakago (tang) usually cannot be seen, and it is not very safe to date a sword based upon the tang. It can only give a hint, because usually the older swords have thicker and darker rust, while on newer swords, there is very little rust and the yasurime can be seen much better. But I have also seen shinshinto swords with tangs that looked like koto, so you should be very careful with that. It is much better to date a sword based upon the workmanship of the blade.
As for the chinese tangs you have shown, you should better not take a white background, because there is not much to see on these photos. You should also better ask in a Chinese sword forum, e.g. SFI or else.