Can I travel from Hong Kong to Shanghai and back to Hong Kong as a 24 hour visa free transit?

No, you cannot do that because this is not a transit, but a return trip. However, with just a bit of creativity, you can enjoy both 24 or 144 hour visa free transit by inserting Macau into equation. In other words, you can create itinerary to look:

Hong Kong - Shanghai - Macau

When you land in Macau, you can immediately return back to Hong Kong in the way you wish.

When you reach Shanghai you will approach transit counter where you will apply for a temporary entry permit to be allowed to exit the airport. This application is free of charge and here you will just present your flight tickets. This is the reason you should have your flight tickets in the form of printouts.