Chickens closed up in a coop for days or weeks without exercise, sunshine or fresh air will get sick especially if the vents have plastic over them.This heater makes the air warmer and makes the ammonia more toxic thats trapped in there.The OP stated while her husband was in ICU she was working...
Their roosts are far enough below the vents that they don't get a draft from these vents! If you've got high winds blowing inside your coop from the vents you can make a frame outside and attach it to the side of the coop to block wind but not air(a foot or so from the vents)
My suggestion is remove the plastic off the vents and leave the heater. The worst thing that we can do to chickens is block the vents and add heat at the same time thanks!