@Peregrine Don't overthink too much regarding your travel. As long as your documents are in order no need to worry. If asked, just answer truthfully. You can also search Youtube for episodes of the series Border Security Australia to get some ideas on how immigration and customs handle random checks at the airports here in Au.
Afaik, BVs are granted only while onshore and you are applying for a substantive visa. So I do not know what your agent is saying na "pwede but no guarantee". But then again, your agent knows a lot better, I suppose, because they do these things on a regular basis.