how do you know the first seller hadn't already swapped out the sangu - I don't know how you would know, please share this knowledge,
Did the other seller sell it with the maedate in your first picture?, dealers in Japan do swap parts around and break up sets for profit, while I don't like to see that it does happen, The hair looks like it is pushed into the tehen (removable plume) in my opinion.
Sangu set is matching, gusoku I understand means matched rather than made to match as you have stated,
The kabuto, mempo, sode and dou are all matched, as are the kote, hiadate and suneate - which is a matched armour