> So learning the Tenketsu attack against one specific person may be something a Sharingan user could possibly do

A Sharingan user cannot hit someones tenketsu as it was stated by Kakashi in ep 47, 5min 4 secs, that the sharingan eye cannot see the tenketsu points. Even if the sharingan eye can copy the movement of the Hakke Rokujuuyonshou, they can't hit the tenketsu accurately.

I need to get out more....