Bugga, I hope the rest of your fish do fine.
I bought a betta in the weekend he's a brilliant turquoise. BUT he has white spot, which I’m not to worried about because it’s very mild. I did know that he had white spot when I purchased him, and I was willing to buy him because he looked so damn good.
I’ve never heard of a death like the one yours had, It would be a lot better understanding it. Hopefully somebody here has an answer.