I'm so sorry you have had such a rough time with these chicks. I am usually the first to suggest Marek's Disease, but I don't think it is the culprit in this case.
The first few deaths were far too young for Marek's. I would say they may have been due to stress, over heating, dehydration, pasty butt or any number of different things. Your next ones clearly had a respiratory problem which may have been viral or bacterial. I also wonder if ventilation/over heating may have been a problem. They appear to be in a plastic tote. Did you use a heat lamp and did you provide a cool side for the food and water. Heat can build up in a solid sided box and the water gets warm and they have no way to cool themselves down. What type of bedding material did you use and how often did you clean it? That can cause issues. I'm not saying that this was the problem, but it can be.... They may well have hatched with a respiratory virus inherited from their mother.
Feces will look normal sometimes but diarrhea is always present the same day
I wonder if you are aware that chickens have two different types of poop, the normal brown/green ones with the white cappings that don't smell and caecal poops which are more runny and shiny and stink and look a bit like diarrhoea.
Can you post a photo of the poop you are concerned about?
The eye colour change may just be a developmental thing or it could have been caused by dust or debris getting into the eye.... Does the remaining chick have this strange eye colour and if so, can you take a close up photo.
The dander may be as a result of a juvenile moult or they may have a parasitic problem or they may have been dust bathing in shavings and have small sawdust particles trapped in their feathers so that when you pick them up you get covered in it, especially if they are French Marans and have feathered shanks. Can you hold the chick over a dark flat surface and ruffle it's feathers and then take a close up photo of the debris that comes off.... if the debris is black, then use a white surface, so that we can see it. People's perception of a lot of dandruff can vary immensely depending upon how scrupulous they are. Have you examined the skin to see if it looks dry or inflamed.
Clearly these birds were not thrifty when you bought them and perhaps your inexperience has meant that you have not recognised problems until it was too late or maybe you realised there was something wrong but did not know what to do to help them. I do think there has probably been more than one issue causing the deaths and they are probably just a combination of bad genetics, bad luck and perhaps some inexperience.
I apologise if I have suggested things that you are already aware of and have ruled out. It is always difficult to figure out what is going on when you are relying on someone else's description and cannot handle the bird yourself or know the level of experience and knowledge the owner has.
I hope your last little bird is a survivor.
Good luck with him/her.