Wow, you're really brave to take on a trip so soon after knee replacement. It's great that you're walking well so soon, so you should be in pretty good shape in five months.
If you're talking about the Aus/New Zealand/Fiji trip, I think the hardest part will be the long flight from LAX to Sydney then changing planes to Cairns. However, the Qantas flight was quite comfortable, even in coach. I get up and walk a lot and had an aisle seat. I live on the east coast so spent a night in an airport hotel before and after the flights. Business class should be even better.
I did not take either of the extensions so cannot comment on those.
I don't remember a lot of walking or a lot of hills, except in Auckland. I have a bad knee and found walking down the steep hill from the hotel painful. A cane or trekking pole would have helped there. Sydney and Cairns are flat, as are Queenstown and Christchurch. Milford Sound is beautiful and the ride from Queenstown and back offers spectacular scenery. There might be stairs in a few places, but your husband and a cane could help you there.
Fiji at the end of the trip is heavenly and a wonderful way to end the trip.
Good luck with your recovery. I hope you take the trip and have a wonderful time celebrating your anniversary.