@avp_manaloto first and foremost, congratulations! 🙂 yeah, if darating siya ng July 5, October 5 most probably last day niya. Pero to play safe, lagyan mo ng buffer of one day or week before just to be sure (i.e., between Sept 28 to Oct 4 departure).
As for coming back, depends sa visa officer sa succeeding entries niya. May mga stories na naque-question sila each and everytime pagbalik after first entry for 3 months (regardless of the day's outside australia after last departure). Pero if it makes you feel any better, meron akong 600 subclass dati bago ako naging PR, and within that one year, 4 na beses ako pabalik balik and in no instance ininterview ako. Ung words lang from them ay "Good Morning/Afternoon/Evening/Day".
Pero curious lang ako, may program si Australia for parents of PR that have 18 (yes eighteen) months multiple entry validity, but they can stay for up to 12 months straight without leaving. Di ba to na-explore or ung nabigay?