Thanks for your reply. I guess they do things differently in Iowa! Like I said, I've never, ever heard this ever discussed before, or ever seen a single parent do this. I'll check with my pedes and FP doc friends, too. UP here, we just adjust the straps to accommodate the outerwear at the beginning of the cold weather season. They should still fit snugly over the child. I have responded when on call to a number of carseat ejections over the years that arrived to the ER still in the carseats, from EMS transport-- and I can't remember a child without a coat in the winter. It's more likely the whole carseat will be ejected with the child in it, than the child ejected out of the seat due to loose straps, now that we have 5 point harnesses standard on all carseats.
However, if we are discussing small infants, say, under 7 or so months of age, then I can agree that the bucket style carriers are different. You can carry them into a warm building to put the small infant in, and they make lots of zippered blanket style bags that you pass the seat straps thru when securing, and then use an elastic blanket over wrap thingie on the whole bucket carrier. But once in a seated, installed carseat, every parent I have ever seen places the child in the seat with their coats on, and buckles the 5 point harness over it.
We can agree to move the discussion on, at this point! Cooper wasn't wearing a coat, presumably, so it doesn't matter. I would, however, strongly disagree that anyone who leaves a child's coat on in a carseat is an irresponsible parent.