Dropped dead -- should we worry?

We have a mixed flock -- 6 year-old hens, ten cornish rock crosses, and about 10 other fancy chicks, all around 8wks old. The Crosses are for meat, and we were planning to have our Sweeney Todd Party this weekend. Last night, we found one of them dead. Slight abrasion on the tip of one wing, head at a funny angle, so we figured it had tried to hop over the 4" lip of the fence, and being a big, fat, clumsy Cross, it tripped and broke its neck. But then this morning, I found another one dead in the middle of the pen, no marks on it. While huge, these two were not the largest birds we have.

So, the questions:
Is it just that at 8weeks, they're ready to start keeling over?
Or is it possibly an infection?

If it's the first, then we should hurry and butcher them as planned, but if its the second, we'd worry about the safety of the meat in the others.

We called the state to get the flock tested and banded, since we need that for the others -- would a problem show up in that test? But the tester isn't coming till next week. And if we ask them specifically about a flock with chickens dying in it, what are the odds they'll say we have to cull the whole flock to be safe? What are the odds they're right? There's a few of the fancy chicks that will be meat later (eg, those without the sense to be female), but the rest we have for eggs and for themselves. They've got names, and children who will cry if someone makes us kill them.

Any answers/advice? Also, anyone near SE Massachusetts who wants to come to teh Sweeney Todd Party? If you haven't slaughtered and wanted to try it out, let us know (assuming we get reassurance to proceed).