Re: Post Mortem Practice
Suspect #1 always has to be varroa. A couple reasons I say this. #1 Sounds like some of the dead brood had tongues extended? #2 It seems a bit late to have a lot of brood (though maybe not in your area) and queens that are still heavily laying late is quite often a sign of mite stress.
Robbing would be a seperate issue resulting from undefended hives. Have you made a real close inspection of the bottom boards. If it is mite related there should be quite a lot of dead mites there. The most likely scenario is that the hives were weakened to the point that the feed attracted robbers which the hive simply wasnt strong enough to defend. Sorry to hear about your problems, hope your other hives fare better.
"Ve are too soon olt und too late schmart."- A nameless German philosopher