Dragon, I've made that same gruesome discovery today as well. For over a week, my orange Platy mysteriously disappeared. Today, I found the lifeless body. Totally white, no color anywhere. Huge chunk of underside is missing. Fins and tail is gone. Partial head is gone. I cant even find the mouth.
No, the male Betas are not together. Well, they are, but divided by a clear plastic. However, female Betas can be together, depending on their personality. Its always a toss together and wait for the female Betas.
Thunder