Availability means there needs to be a free seat. You can not do stand by. Depending on day of week and the time the planes are often full and can't change. Given you are doing WLG-ZQN there aren't too many other flights to change to, so look at the schedule to see if it is going to be worthwhile. They are generally most useful on the main trunk routes where there is one flight an hour so an option to change.

I don't think you can change the routing.

With flexichange have to change before checkin closes for your booked flight. Need flexirefund if you want to change flights after missing checkin, I think.

Can make the changes through the app, in most cases.