It's not really any different than any other airline. If there were no window or aisle seats available when you chose to buy a middle seat, that's because other passengers already took those seats. They don't cost more, and will only open up if someone cancels or upgrades (if available) to a higher class. I would check at a little after 24 hours before take-off, when Gold Elevate members are auto-upgraded to MCS or can pay-upgrade to F.

I generally check in online on my phone and then save the boarding pass to Apple Wallet.