Hi
Sorry to hear of your losses. There is a lot of joy but also some heartache in keeping chickens, as there is in life in general of course.
I agree with the advice given by @Yorkshire Coop but would like to add that lice often thrive on sick birds that are too unwell to bother dust bathing/preening, rather than the lice being responsible for their sickness. Lice are an opportunist pest and more of an irritant than a serious health hazard..... unlike mites which suck blood. The photo of your bird appears to be lice and you may well find feather shafts caked with eggs like those in the photo YC posted near the vent and under wings.
Providing a good dust bathing area with wood ash and sand or dry soil will help them to keep themselves free of lice, if you don't have one already, but once you get an infestation, then you need to actively dust them with a louse powder once a week for three weeks to catch each new batch as they hatch.
As regards the hens that have died, there is some underlying problem as hens should not die from moulting or lice. Look closely for other symptoms. You may be able to send off a poop sample from a sick bird to a lab to be examined for worm eggs or coccidiosis. In the UK this can be done direct to the lab or with a veterinary practice. I don't know where in the world you are located but I imagine there may be similar services where you are. Also, if another hen dies, it is really useful to get a necropsy/post mortem done so that you can hopefully identify the illness and treat/prevent your other chickens from suffering the same fate.
I hope your flock are looking and feeling better soon.
Best wishes
Barbara