Hold each egg up right next to your ear and you will hear if they are alive, as they would be close to hatching now and making rustling, cheeping sounds. The broody might also be humming to them if they are alive. Also, give them a slight shake (not a big shake) and if you hear sloshing then you know they are rotten.
If they are truly all rotten/dead, then the broody will give up on her own, as she will not be hearing them cheeping either. She does not need to be put in the naughty cage, just let her realise the situation herself and she will be back to normal without any stress. Don't deliberately let her brood again this season.
Often eggs sent through the post get badly shaken about so that the yolk gets tangled up and any embryo damaged. They may have been perfectly good when sent, so not necessarily the breeder's fault. Try to source eggs locally next time and arrange the pick up carefully to account for distancing.