It greatly depends on the school you are looking at with respect to applying/tours etc.

Most schools have enrollment open only the year before kindy but a very select few have waiting lists years before (it isn't like day care where places are scarce).

If you are looking at the Catholic system you'll likely find that whether or not you and DH are Catholic, you will be offered a place provided you are agreeable that your child will be taught Catholic doctrine (I was raised in the Catholic system as a Catholic, my DH is agnostic and I am non-denom Christian. DS is in a Catholic school and we needed to sign a statement acknowledging the Catholic teachings and he came home partaking in Lent masses etc)

There are the elite schools that charge very high fees and generally they are the ones that require a long waiting list. If these are in your consideration, your best bet is to put a call into the school or hop on a website to check out their enrollment criteria and procedures. Whether you are alumni doesn't really matter these days, it's more about making the commitment to meet their fees.

I don't think that you are overthinking things, I think it's wonderful that you are so aware of potential challenges before they are in front of you. This is the best way to manage anything!!

I don't have any input about the schools in your local area (I am a teacher and mum in NSW) but my best advice is to look at all the schools, make contact regarding tours/applications and then wait until the year before to make a firm decision.

A lot of things can change in time and the principal has a lot of influence. That said, if you want the opportunity to choose, you might be asked for a non refundable deposit on application. You can always apply, sit the interview/assessments etc for all schools and then have choice at your fingertips just prior to your DS3 starting kindy.

We did that with our children with the 3 main schools and then we decided in the months leading to kindy based on the schools at that time. A little expense involved, but worth it for choice.