Re: Sharingan awakening plotholes.!!
Normal Sharingan 1/2 Tomoe is unleashed with severe trauma. Eg. Sasuke witnessing his entire clan dead, Obito about to lose Kakashi his best friend.
3 Tomoe is unleashed when a Uchiha is pushed to their limits eg. Sasuke fighting 9 Tails Chakra Naruto.
Mangekyou is unleashed when killing your best friend. Itachi "killing" Shisui. Sasuke killing Itachi, Madara blaming himself for his brothers death.
Eternal is only avaible in direct blood relatives.
These are how, not why.
The Sharingan evolves with loss of love and a increase in trauma/ stress. This is because the Uchiha treasure love so much that when it's lost it causes the special chakra in their brain to change their eyes according to the level of love lost.
Hope this helps.