Only the first. You are using the word away incorrectly.
If you throw the ball away, you have disposed of it, put it in the garbage, gotten rid of it. The garbage collection people have come and taken it and you will never see it again!
1. We generally use adverbials after "throw" as in "I threw the ball into the river" , "I hit the ball outside the stadium" etc., so why can't we say "I threw the ball 500 metres away" ?
2. Is "500 metres" in "I threw the ball 500 metres" an adverbial ?
why can't we say "I threw the ball 500 metres away" ?
Technically, you can, but it doesn't mean what you think it means, and even then it's a stretch.
It can be stretched to mean "I threw the ball while standing 500 metres away from here". I don't recommend using it, even with this meaning. It's not idiomatic.