Sorry for your losses.

I cant give any advice about the sickness. Never had something like what you describe in my flock. Hope the others help(ed) enough.
Soon after I started with chickens I heard from others who got a disease in their current flock after buying new pullets. Quarantining for several weeks is very difficult and you need a second coop and run to do this properly. This is why I prefer to buy hatchery eggs if I want to expand my current flock. (My problem always is dealing with the roosters).
I heard about the heatwave in Spain, Italy, Portugal and France. Today the heat arrives in the part of the Netherlands where I live too with an expected 33°C for 2 days. Which is a lot better than the 46°C they had somewhere in Spain. Unfortunately we are going to have more of this extremes in the future bc it seems impossible to convince /pursue the directors and shareholders of oil companies and energy consuming factories/businesses to make the necessary change. (Eg. Exxon mobile is even pursuing people who try to make a statement for a change).
Governments are listening too much to the ultra rich and are afraid of an economic disaster. Not realising enough that every year we go on with high consumption of fossils makes it worse for the future. The news(papers) are of little help and lots of media ignore it or spread misinformation and get paid from the from angle.
Most consumers who are worried have the feeling their contribution is so tiny it doesn’t matter what they do. One of the peoples action now is to buy air conditioning without solar panels to make it even worse.
Many people talk about the heat but not what caused it. They are very worried about foreigners coming to live in their town but don’t seem to care about the unpredictable climate disasters who are coming our way. Its making me very sad that the people in this world act like frogs in a pot of water with rising temperatures (famous experiment, by a German physiologist).