
So sorry to hear such a sad whelping story, may they rest in peace.
By what you are saying the pups had been dying for a while as some were smaller so it didn't happen during the labour, surprised the vet didn't suggest a c/section when there were still heart beats after so many stillbirths.
To prove Herpes one or more pup would need testing.
Ideally the spay is done half way between seasons, my girl comes back into season when her pups are 17 weeks old so I would wait till 12 weeks after that season, ask your vet what he suggests.