Did everything, from the stick arms on cake to the Scratch in John's session happen in one day?

No, it happened in SLIGHTLY over a day, the Reckoning gives 24 extra hours of ‘okay now hurry the fuck up’, which means there was a period from John’s introduction to [S] Descend prior to the 24 hour countdown. Dave played the Session for weeks to help Terezi contact them in the first place and then help Jade get most of the frogs she needed.