A couple of worms walking around on the underside of the lid is normal. That by itself doesn't mean anything, they just like to travel across any surface that's a little wet from condensation.

400 worms is not very many. Are you sure they're all gone/dead? They could be hiding very deeply.

I turned over almost the whole bin and none were evident, nor did I see any cocoons/capsules?.

If the worms died, it's hard to know what killed them from your description. Anaerobic decomposition in food and bedding creates a bad smell that is pretty hard to miss, but you said there was no bad smell, yet you had food in there that you thought was anaerobic. If it didn't smell bad, what made you think it was anaerobic?

What is the source of the cow manure you used? Any chance there was animal medication in the manure, or was it mixed with any bedding material that might have had chemicals added? If it heated up, were there cooler spots in the bin for the worms to move to?

The food was in a plastic jar until it broke down, not stinky just really wet and slimy. It consisted of mango and peels, watermelon rinds, tomato peels, lettuce and broccoli stalks. I bought the manure from Lowes so I'm assuming it was sitting long enough to be clear of any meds.