Not doing bio in the hsc hasnt really impacted my learning in med. A lot of the bio is fairly easy to grasp and not really a major component of the course, at least not the bio you did in your hsc. So not doing hsc biology doesnt really disadvantage you
It is good to have multiple reasons, and a passion for science is a very viable reason
its beat just to be entirely honest with the interviewers
I dont think they are as "intense" as people make out. Different unis have different types of interviews. UNSW has a panel of two, whereas uncle and uws have MMI interviews