I had the same reaction terracosmo. Kakashi was an uchiha and no one knows it in the village, he left uchiha clan because of his deformity and none did anything (these clans are capable of killing their own members to prevent their bloodlines to fall in enemy hands, and they just let a sharingan guy to run away from the clan and live on his own...50 meters away).
Of course kakashi hides his eye because of his childhood trauma, created by his deformity, thats why he made a quite big scar in his sharingan eye, and hides it, and every time he opens that eye is in sharingan, that is also part of the trauma, he cant un-sharingan his own eye.
Also its said that is true that he gets tired by using sharingan because he hasnt uchiha blood(he always ends exhausted after using sharingan), this must be from that trauma again, that he consciusly forgot his uchiha past. He left uchiha clan at the age of 5-6 but at that age he already knew about mangekyou and how to obtain it, he killed his friend and no one cared about the strange death of his friend, when he is an uchiha (so.. able of getting mangekyou by the way of killing your best friend, this isnt suspicious at all).
Why didnt he use it against itachi¿? He liked to be tortured almost to death, maybe, he couldnt use mangekyou because of chakra needed to it... he has bad time with the 3 comas sharingan.. with mangekyou... but oh well he can use mangekyou but only when his chakra is full and concentrates, then some genins come, look at his feet throws a lot of kunai and kills him because he runned out of chakra and couldnt mold any to move fast... what a great jutsu!! you killed your friend to be defeated by 2 or 3 gennin.. obviusly a genius.
With the clans thing... i supose that clans dont marry between them because they look like they have 50-100 ppl so the chances to get deformitys and so is very high, konoha looks like our culture were almost always the wife goes to the "clan/family" of the husband. But what happens if u get a hyuga and uchiha married, hyuringan? this part of the histori has a lot of flaws, kishimoto makes mistakes too so dont think too hard about it. I have been thinking taht maybe they are banned to marry with other clans and only could marry with ninjas or normal citizens without bloodlines limits. Dunno.
Bye!