a) I don't think its necessarily a prereq to have done volunteering, its just for highschool students there not really much else as you can do. As a HCW worker who is able to express how this role has influenced them you would be more than fine. Especially if you tie your experiences into the JCU med program's goals (rural, atsi, tropical, etc)
b) No I wouldn't say so. In fact if you're able to explain how this experience has made you a better person, and more focused on the JCU med program's goals then I would think it's a positive experience. It was a hardship, you powered through it, and you're better because of it.
In saying that I wouldn't so much right it in a "sell yourself" way like i can show commitment to the med program but in a self reflective way talking about how it shaped your perception of difficulties faced by regional/atsi communities where teen pregnancy rates are higher for example, or maybe there isn't as much access to childcare services, or getting to uni is 10x more of a hassle then it was for you, etc... That's what they want to see. They have a plethora of applicants who are able to face hardships to complete the program, what they're looking for are applicants with compassion to the issues that they feel are important
Just my 2c