The problem with o/w tix on these international flights is they are often pushed up to Y fare basis. Normally they'll be priced slightly cheaper than the r/t.

If you're so concerned about getting the 600miles credited then suck it up and pay the o/w price.

If you're concerned about the cost than see if QR works for you (they don't fly everyday).

Unfortunately you'll need to decide one way or the other.