The best Sparpreis price isn't going do me any good if you missed the train due to flight/immimgration/baggage delays etc since you can't change it on the day of departure and it's no value. Once you get the walk-up fare, the "at a fractional cost math" goes out the door.

You wrote yourself "Simply give yourself a hour or two extra after your transcontinental flight to get from the airport to the train." So suddenly the 1:19 journey time becomes a 3:19 journey time?

Please reference the trains you are train/price you are getting your numbers from.
The only 29EUR option I see involves at least one connection with a journey time of 2:45 or more hours. So add on another "hour or two extra" to qualify for the Sparpreis becomes a 4 hour affair.

Is the train a fractional cost when travelling with a small party size on Sparpreis and everything goes according to plan? Yes.

Is the train a fractional cost when travelling with several passengers or if a travel delay happen? No. The train might be comparable (again depends on the number of passengers) but it could also be significantly more expensive.