And breathe ... two ... three ... four ...
Just relax. You're not going to be rejected just because you're not sure which agent to send it to. If this agency's guidelines are unclear that's their fault, not yours.
'Initial contact by phone or email' is a little unclear when there are also specific submission guidelines, so hedge your bets: send just the query letter by email, with no sample text, and it's also okay to say that you weren't sure how much to send initially, and you're happy to send more if required. You're not going to be rejected for being thoughtful and polite.
In terms of whose name to use: use the agency's name. 'Dear [agency name] ...' The agency I work for gets enquiries about which name to use but, really, it doesn't matter to whom you address the query - it will get read regardless. Just don't say 'Dear sir' unless you happen to know it's run by men. The Australian agents are overwhelmingly female in number and nothing sets the teeth to grinding like a 'Dear sir', because it indicates a complete lack of attention to any sort of known detail about Australian agencies.