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I do not think you have a ventilation problems at all. When I read this instantly soggy or mushy chick syndrome came to mind. It is classic symptoms.
You have a bacteria infection happening at some point in your eggs. The foul smell is the alarm bell and the other problems with the hatching eggs solidifies the diagnosis for me.
How have you cleaned the incubator between your hatches? Every part needs to be sterelized. The places you can't spray with bleach solution needs to bake in the sun for several days before you even try to hatch again. Even then i would be cautious. If you have to take it apart and hand clean the electronic pieces. Submerge or completely douse in bleach water all of the other non-electrical parts.
There is a thread here somewhere in the archives where dlhunicorn posted the complete scientific diagnosis for the condition. It is a bacteria that causes the chicks to seem as if they are water logged, they can't absorb their yolk sac and the bacteria is concentrated in the belly and causes the stench.
I'll see if I can find it.
Here is one thing I found -
The disease is prevented by careful control of temperature, humidity, and sanitation in the incubator. Only clean, uncracked eggs should be set. If it is necessary to set dirty eggs, they should be segregated from clean eggs. Sanitizing detergents must be used according to directions if eggs are washed. Time, temperature, and frequent changes of water are as critical as the concentration of sanitizer in both wash and rinse water. The rinse should be warmer than the wash water (which should be warmer than the internal temperature of the egg), but should not be >60°C.
The incubator should be cleaned and disinfected thoroughly between hatches.