Welcome to the forum.
You have done a good job planning , and you can do all those things.
A few comments -
If you go direct from the airport, you should save time going to the Rhein falls. At least, you are closer than you would be if you went into Zürich. You can leave your luggage at the hotel, in lockers at the train station, or at the left luggage office, if you wish to return via the airport.
There are two trains per hour from the airport to Luzern. One stops at Zürich main station, the other requires a change at Zürich.
I'd not plan on a lot of time at the falls.
Ooops I see that Annika answered while I was writing.
She's right about the trip to the Jungfrau, but, if you like long train rides, you could do it. The issue will be weather. There is not a lot to do there other than ride the cograil - great views if no clouds, and, take the cog rail up to Jungfraujoch. Same comment.
When in late December? The alpine resort regions come alive on approx. December 15th. Before that, many services and hotels are closed. After that, they are very busy for Christmas.
This will help you with timing and routing:
The latter map is useful for travel. If you check "Traffic" under points of interest, and zoom in enough, icons will appear for train, bus, boat stops. Mouse over them for some schedule information.
A half day may be a bit much, but sunset is at around 1630, so you will have only a short afternoon.I like to walk around the old town in Zürich and definitely think it is worth spending a few hours. You'll have a day in Luzern at the end of your trip.....
Be sure to walk the Bahnhofstrasse. The lights are striking. There is a Christmas Market in Zürich main station.