First, I have never hatched Calls but here are my thoughts if it helps.
Are you washing the eggs before setting (you shouldn’t be)? Have you sterilized the incubator between hatches? Bacteria would explain them dying at different stages and being weak at the end.
When you raise the humidity...
Maybe it’s just the picture but that head looks chocolate brown to me, not black. If so, maybe lavender (or is it lilac) which is blue and sex-linked brown.
I agree with you, those eggs are not fertile. My drakes often take a while to get going in the spring. Although I am getting eggs right now, mine are not fertile either. Last year, it took until May before my eggs were fertile. So, your drake could just be slow.
Or, I had one drake become...
Ha ha, no they would not be monstrosities, I’m sure they would be adorable. The Khaki hen’s babies would be gray and look similar to a Mallard. The Magpie hen’s babies would be colored like her.
Sure, I love genetics, but I need to know more about your mixes. Do you know what they are mixed with? Was the Khaki a hen or drake? Photos would help too.
Also beautiful! I had a Khaki x supposedly Appleyard cross that looked similar and had to be M+/md as he also had Mallard pattern as a duckling and facial stripes during Eclipse. This was him:
Soon, very soon. I got my first egg of the spring this morning (Dutch Hookbill’s though). I live in Tennessee so I may be ahead of you depending where you live.
I can’t answer you about Rouens as mothers. I have always used an incubator for my different types of ducks. That way I still get...
I have added sealed gallon jugs of hot water to my coop. It won’t hurt as long as it is sealed so you aren’t adding moisture. It will make you feel better if you see it isn’t frozen solid in the morning too.
I keep 2 seed starting mats under my 3 nest boxes (large dish pans)🤷🏼♀️. They are attached to an automatic Christmas lights timer which goes on at dusk off at dawn. I added the mats to stop frozen eggs but it gives them a little heat (but I am not as cold as you). I also added a remote...