Austria + Munich/Wurzburg

That’s already a well-prepared itinerary and can work out as planned. Let me add my thoughts:
Your Hallstatt and St Wolfgang day will be a long day spending 5-6 hours in public transport.
https://fahrplan.salzburg-verkehr.at/ helps for detailed planning.

For the Innsbruck/Seefeld section keep an eye on the weather forecast. Maybe you turn it around and stay in Innsbruck (as it has more bad weather options) and do Seefeld as daytrip?
For your skiing/snowboard/tobbogan day you could also consider also https://nordkette.com/en/skiing-snowboarding.html

On December 24th shops will close around 1 or 2 pm. Christmas is typically celebrated at home with family. The Ice Skating Rink at Karlsplatz was not open last year. It could be, that it will be open till 4pm on Christmas Eve (it’s not yet scheduled).
Also most restaurants will be closed, thus you should reserve your dinner (e.g. restaurants in larger hotels will offer special Christmas dinner) well in time.
Could be quite interesting to stroll around in the evening twilight in an almost empty inner city.
Of course churches will welcome you and there are enough people in town who don’t celebrate Christmas and might organise other events that you could look for in local newspapers or ask for in your hotel.

Lived in Wurzburg and was not impressed with Marienburg. The view of it from the town is much better than the view of the town from the fortress. But that's just my opinion. With only one day there, my suggestion is that you spend your time at the Prince Bishop's Residence (especially the small chaple on one end of the main building) and wandering about the old town and several of the churches. A google search should turn up more than enough interesting things to do other than the fortress.

Thank you both for your kind advice! The Salzburg route planner looks super useful - will definitely use it to plan my day properly!

I am considering to cook up a Christmas dinner during the quiet nights (fingers crossed hope my food will be good)

Hmm, if the view from Marienburg Fortress isn't really anything to marvel at, is there any particular castle/fortress I should visit (preferably somewhere near the places that I'm visiting) ?

Your Hallstatt and St Wolfgang day will be a long day spending 5-6 hours in public transport.

This is a bit of an over-estimation, in my opinion; it is doable in less than 5 hours net travel time.

The said route planner for the Salzburg area will give you the information how to minimize travel time. The fastest route would be as such:
Salzburg - Attnang-Puchheim (express train), Attnang-Puchheim - Hallstatt (regional express train), Hallstatt - Bad Ischl (regional express train), Bad Ischl - St. Wolfgang (bus), St. Wolfgang - Salzburg (bus).

In Hallstatt there is for every arriving or departing train a ferry across the lake as the town and the train station are on opposite sides of the lake respectively.

In Bad Ischl you might consider a break for a short visit of the town which has become famous as the place for emperor Franz Joseph's summer vacation for more than 50 years. At least you should go to café Zauner and buy one of the unique "Zaunerstollen", a kind of chocolate cake only available there. They have ones in a sealed package so you can bring it home to the US safely.

Since you seem to really be interested in fortresses, don't let my one opinion keep you from going to Marienburg. It does have an appeal for those who are into those things, and is definitely worthy of a visit for some. Just not high on my list of thing to do. Since you will only have one day in Wurzburg, and there are so many things to do in the old town, you need to plan ahead so you'll be able to get the most out of your visit. Do a google search for "Things to do in Wurzburg Germany". There are several articles detailing more than enough things to help you have a great time there.