"Ma" is always used at the end as an interrogative auxiliar
Yes, and we don't need exactly an interrogative punctuation to determine that as an interrogative sentence. Just need "ma" in the end of it.
Why is shi not required here
It's just a rule. Using "ma" you don't need to specify anything else.
Why not " 你李吗？"?
Never mind, I got it :)
Why no da too
Why is de (的) not required? Is the last name word (姓) a verb?
Yes is a verb in 汉语