New plasterboard

It depends on how much is snapped off and what background board is going over, if it’s onto brick or block wall bonding coat plaster can be used to fill prior to skim coat, if it’s onto timber/metal studs would beat to get a new board. The final skim coat is only 3mm thick and won’t be able to fill heavy undulations so it is important if you want a good finish to make sure board work is done right, most good Plasterer’s will tell you to remove and re-do bad prep work / boarding rather than leave a bad finish. than

The best thing to do would be to put some bonding into the area and patch up with skim. Pretty much as you said. Don’t forget to pva over the bonding before skimming

It sounds like a damaged plasterboard around a socket outlet from the description. Just get some drywall adhesive and overfill around the damaged section and this may take a couple of applications depending on the extent of the damaged portion, allowing to dry between coats. Some fiberglass scrim bandage may be needed to eliminate cracking. Then sand it back until smooth or leave it slightly recessed and skim over with gypsum plaster.

Hi I would personally use expand foam to fill any minor void and cut it back when dry then scrim over foam b4 skimming I hope this helps thanks

Yes, you can do as you suggested.just make sure you scrim tape the joins between your filler and plaster board

Depending how big the void (gap) behind board if it more than a couple of inches use foam as suggested because material will keep falling behind and if not to big bonding,hardwall depending on background behind make sure you level to board and socket case and scrim.