I looked up staphylococcus aureus and found this link pretty helpful :I looked on the government page and it also says the PenG should not be used in laying hens, only approved for broilers in Canada. So I guess the vet feels this is the only option, and hoping the residue will not be present after 10 days.
https://www.msdvetmanual.com/poultry/staphylococcosis/staphylococcosis-in-poultry
I can see why your vet chose to go along with penicillin. If you don't feel safe you can always put the eggs away for a longer period. I believe it's more of a public health measure to contain the growth of resistance to antibiotics.
One of my work colleague actually nearly died from this some years ago and it was incredibly fast. She was probably only saved because her husband was a med in the ICU.
I'm afraid if the four weak hens are infected they don't stand much chance. Then again if they're not getting worse maybe it's worth trying the whole prescription.
I don't have access to a vet where I live and most of chicken's medication is also restricted in my country. Sometimes however the same active substance is not restricted for other animals like cats or sheep's, and sometimes it's even sold with no prescription for children. A few times I used those when it was life or death, and had help to calculate the exact dosage for a hen relative to her weight. The dewormer I used, and vitamin K. when my hen ate a poisoned mouse were both children medications. Not a choice I would recommend though except in case of emergency!
I hope the treatment helps your flock, do let us know!