I'm neither a vet, nor a photoshop expert, but the scars on the puppy remind me of similar ones which out late Tina had when she was a puppy, i.e. in 1993. They looked very badly, but turned out to be just symptoms of allergy. It took us quite a while to recognize the allergen, but when we changed her food, after a couple of weeks or months (I don't remember now) the scars disappeared.
It's quite common that pups' adaptation to new home germs/food/fabrics/ cleaning chemicals etc. takes some time and they have various symptoms - like diarhhea, scars, scratching, etc. Still, one can hardly blame the breeder for this...
Erwan have you tried changing the home cleaning detergents to
very mild ones (e.g. liquid for intimate hygene), washing the pup's blankets in washing powder for babies, eliminating chicken and diary products from her diet, giving the pup calcium, etc.?
Sometimes common sense and simple solutions work best. Good luck!
And one more thing: happy, healthy puppy is exetremely messy and this is just a symptom of natural learning process!