Re: Please help diagnose what this is... Video..
AR Beekeeper gives good advise.
Just to add to it, you do not at any time mention eggs, we need to know if the queen is laying normally.
the brood, the last 2 brood frames you pulled had chilled brood. My guess is because there are not a lot of bees in the hive, and if you had a cold night, the bees where forced to cluster and could not cover all the brood so it died. The first brood frame you pulled looked more like mite damage to the brood, but just not quite possible to tell from the video. My suggestion would be give it a couple of weeks for the bees to clean out the dead. After that, if you still see dead larvae like this, on a comb the bees are able to cover, it's a mite issue & would be a good idea to treat.
The queen cells, don't know as could not see if they had larvae but the second one seemed further developed than a broodless cup. Could be the queen has been introduced and the bees built some cells while they were temporarily queenless. If the queen is laying eggs normally they will likely destroy the queen cells or you could do it. If she is not laying do not destroy the queen cells.
The bees look like classic italian with a small bit of mutt, nice docile bees and should do well for you. But these bees tend to be mite prone and in a couple of weeks if there is still mite damage you may have to treat them or they could struggle & you eventually lose them.
44 years, been commercial, outfits up to 4000 hives, now 120 hives and 200 nucs as a hobby, selling bees. T (mostly).