I understand your concerns but you are underestimating your chickens, they are way hardier than you think. I used to keep budgies and I had an outside aviary, there are many many sames and similarities between bird behaviour including chickens. Your chickens huddling and cuddling in a corner is good, positive and what they do to keep warm especially in sudden climate change. As you know ventilation is essential however it is vitally important that there are no drafts or winds blowing directly on your chickens, - birds, including chickens, create heat by puffing up their feathers to create heat, by puffing up their feathers they trap air, making air pockets which is heated up by their body temperature, Drafts and winds interrupt this process and make it impossible for the bird to insulate itself this way. My run is such that on having winds so strong our garden furniture was being tossed around the garden like it were paper but my run so wind proof I could hold a candle and it wouldn't even flicker. There are many videos on youtube posted by poultry keepers giving advice on how to keep and help flocks through winter, and there are some really good videos posted by people in Alaska at its coldest, it really might be worth your while to give them a look.
As for your ducks, clearly this is a hard time for them, flocks will always be effected when they lose one they are bonded with but survival is an instinct to them and they will adapt. Here is a link to an article you might find helpful :-
https://www.hobbyfarms.com/6-winter-boredom-busters-for-ducks-geese/
plus there are many forums on here - how to keep ducks entertained during winter - many suggest spending as much time with them, walking around with them - imo it would be beneficial to you to make your own thread about this subject and gleam from the responses you get, the suggestions which suit you and your flock.