Generally, if not taking the D trains or G trains, you can get off at the stopover station but there is a limitation of the staying days. And also, you have to handle the transfer procedures at the stopover station when you continue the trip, which would be troublesome. About the details, you would better consult the staff at the train station, as sometimes the regulation varies from different trains.

Yes, you can. But you must buy the ticket from Guangzhou to Jinhua or Zhuzhou, and stay there whatever you want, and buy another ticket from the city and to Shanghai