Generally the only weekday daytime DJ's that move slots to cover other shows is Scott / Chris for Grimmy and Adele for Clara. Though Annie has covered Clara on occasion.
Cover for weekday DJ's generally comes from the above or from weekend DJ's. In the case of early breakfast Yasser or Jordan.
Cover for the weekends generically doesn't come in the form of weekday daytime DJ's except when the charts used to be on a Sunday evening. Cover for this is usually Yasser, Jordan or Adele on occasion.
It would seem that it is potentially listed in their contracts, especially in Adele's case, probably a sweetener for taking the early slot, but someone else would probably confirm this.
Arriving for the show, Adele is only about half an hour before, she always used to post an instagram? Or snapchat? video in the taxi on the way in.
Preperation for this is most likely done the day before by herself / her team. Just like most breakfast show features when Scott is on are pre-recorded the day before like RONR, rather than just a few hours before.. though bangers is usually live.