Why ēdrus and not ēdrusi?
The pronoun 'skorverdon' the subject of the sentence. It can be more literally translated as 'what number' since it comes from words 'skore' and 'verdon' and acts like a noun in a sentence. It is singular, so the verb is singular.
I don't understand why it's valoti and not vali?
I think the literal translation is like "what number / of the men / is sleeping", thus the genetive.