Yuck. It sounds like everyone failed you. It's disappointing enough when a mechanical or other cancellation happens. It's a kick in the gut when they utterly fail at service recovery and shove you out to fend for yourself. I'm sorry it happened to you, especially on a special trip.
Draft the complaint now, take good notes about who failed you and how. Then put it away, and enjoy your honeymoon! Don't let them ruin any more than the first day of it. I'm optimistic that you'll find someone to switch seats so you can be together on the UA flight.
Dust off the complaint after you get back, pare it down to the basics of what went wrong, make a reasonable demand and send it. Maybe you'll get to have another mini-honeymoon later this year, partially on US's dime with a voucher.
P.S. -- After you've finished sorting out the outbound mess, and have rebooked whatever you need to in Oz, take a few minutes to triple-check that nothing happened to cause your return reservation to get fouled up.