No, Harry Potter didn't kill anybody (Voldemort killed himself with a backfiring spell). Dolores Umbridge wasn't at the final battle, she was later imprisoned in Azkaban for her crimes against muggle-borns.
Not known, but she did fight in the Final Battle, so it is not impossible.
No.
Yes.
There isn't anybody in the Harry Potter books called Henry. Or "Herny".
Harry Potter never dies. Voldemort attempted to kill him in the Forbidden Forest in Harry Potter and the Deathly Hallows, but failed to kill him.
Not during the series.
No, Ron doesn't kill Harry.
James Potter did not want to kill his son.
I am not aware of any Lydia in Harry Potter - nor of anybody with a similar name. The closest is perhaps Lily - Harry's mother.
Harry doesn't kill anyone. Unless you count Voldemort. But he still didn't really kill him Voldemort's curse rebounded on himself.
Why? For the same reason that anybody else eats.