If there are piggies that are as yet unaffected, I'd separate them immediately and at all costs. Whatever is going on, it's obviously contagious and terminal, so containing it would be my first priority.
In my experience, infections (whether they are bacterial, viral, parasitic or fungal) are treatable, but only with a correct diagnosis. Viruses are by far the hardest to tackle, but I've seen piggies recover with the correct supportive and anti-viral care. Really, I wouldn't give up on them yet - there's hope, especially considering that they're not declining that rapidly and there are days between the onset of the symptoms and the time they pass away. This means their immune system is fighting it, and the correct medication can make the difference between life and death.
I don't think it would be possible to treat them without a conclusive diagnosis though, which is only possible by a vet. I'm fortunate enough to live in a country where quality vet care is financially very accessible, but I know vet care fees can be expensive in the US. Would a payment plan/paying in installments be possible? Or an alternative way to fund the piggies vet care, like a loan, etc.?