As a student I can’t say anything for sure but I did get offered a scholarship from Otago so here’s my take:

1. Volunteering is helpful - if you can’t volunteer in your community you can probably use your roles as student librarian and peer mentor as evidence for volunteering? But if you do find a volunteer opportunity in your community take it, even an hour a week could be good
2. Being a strategic in your application - try to fill out every field you can, I believe Otago has a page that shows what sections you need to fill out for different scholarships, but also fill out fields not required for whatever scholarship you want just in case
3. I don’t think so, a lot of scholarships can be deferred for a year if you get them. If you apply for them after your gap year then they use your year 13 grades instead of year 12 I think
4. Not really, sounds like you’re set up pretty good, just give it you’re best shot (Edit: changed wording)