Or even just a few days... Any Sugar Syrup will surely freeze right? How are these bees going to survive with no comb built and nothing to build comb with?
Syrup will not freeze at the temperatures you stated. Adding sugar to water lowers the freezing point. I actually left a few mason jars of 2:1 behind in one of my yards this winter and they did not break.
If you feed inverted jars on the inner cover I would not be concerned with freezing. I think the key here is that these are new beeks, I'm sure they are ready to watch over like a new born baby. So you can use methods that may not be feasible for large commercial-like installs. I would suggest putting a warm pint/quart of syrup over the inner cover hole daily. They will suck down warm syrup rather quickly and it is the quickest and easiest way for them to get sustenance.
I'm thinking possibly Just sugar on Newspaper with a shim?
I wouldn't do this. It takes them a lot of effort to make sugar useful. Where are they going to get the water from? Overwintered hives can use moisture given off by honey consumption.
Feed 1:1 in Mason Jars over the inner cover with an empty Medium then top cover?
I think this is your best bet. Give them warm syrup once a day. Key is warm, don't just look to see if they have syrup as they will not take cold syrup very well, if at all. Worst case just rotate between two jars if there is a long cold period.
I have installed packages in the middle of a blizzard and did not have any issues. Remember, they have no brood to care for, so they can cluster right under the feeder lid. Chances are they will only have to deal with this for a few days at most without getting a warm period.