If you take the Capitol Limited from Toledo you would be in a Superliner, whereas the Lake Shore Limited uses Amfleets and Viewliners. Either way you would be in a Superliner west of Chicago.
Also, keep in mind all three routes are currently running tri-weekly, although they are all scheduled to be in Chicago the same three days to allow for connections (Monday, Thursday, and Saturday).
If you decide to take the South Shore Line, that goes into Millennium Station, which is on the other side of the Loop from Union Station. If using public transit, take the Pink or Purple lines from Washington/Wabash to Quincy. It's also walkable in 30 minutes or so.
As to the quarantine, technically you shouldn't do anything outside the station if your state is on the list at that time. However, enforcement is entirely non-existent. At the moment, all indoor seating in the city is closed although that might change by December.
Weather in mid-December in Chicago averages highs in the mid 30s and lows in the low 20s, but it wouldn't be unusual to be 20+ degrees off of the average.