I love Maddy but no eyes susanoo and then Kakashi unlocking PS
The chakra for the sharingan is located in the stem of the brain.
The chakra is manifested within the eyes but it doesn't come from the eyes.
Yin is spiritual energy it comes from the mind.(The brain)
Madara already unlocked the MS
Now if he used genjustu (You know something that actually requires eye contact then that would of been a problem)
Kakashi didn't unlock anything he didn't even have a sharingan anymore
Obito entered his body as a soul by using his dimension hopping eye technique to ...hop dimensions.
Impure world captures the soul and anchors it to the real world using a body and it can use the souls justu
Obito just used the most straight forward and pure version of this by directly transferring his soul into kakashi (because obito no longer has a psychical body anymore)
You just need to pay a little more attention