Thanks for all the responses.
Yep, I think I just had a perfect storm of bad conditions hit this hive one after another. I still think the main culprit was the back to back pesticide die offs which weakend the hive and allowed pests/disease to get a foothold. I thought about adding frames of brood/workers from my other hives to help build it back up, but I was afraid that the pesticide had made it into the stored food and what I introduced might die as well.
When I realized, late, that I was queenless, I tried to give them a new queen, but unfortunately that emergency queen was a dud: just good for the frame of brood in there now and then she took off. I can see Finiski's point about how, if this hive did have a big mite load, once some new larvae was around, the mites rushed to it and really did a number on what is in there now. It has just been one bad situation after the other. Now it is too late in the season to do anything. If I had a strong mated queen I guess I could try to overwinter them in a nuc, but without a queen in mid-Nov., they are goners.
I have kept bees for about 10 years and never lost a hive so this is new to me. I am using this as a major learning experience.