LordGarlic, 26Jul is the first day of the second semester of a few universities in Melbourne. And that might explain why you find it hard to get a seat in S class.

If I were you, I would get a confirmed S seat on the earliest date and have the booking ticketed first. Once you got the ticket number, ask your agent to put you on waitlist on 22Jul and ask them to use their little trick to get you a seat on 22Jul. By doing this, you guarantee you have a seat leaving for MEL while not missing the deadline of your airfare.

I would say the rumour you said might be true. My travel agent has always have some method to get me a seat in those very busy dates.