I understand your concern. My neighbor does things sometimes that make me very nervous like letting her kids play with scary tools and dangle off second story decks. I got to the point where I didn't feel comfortable allowing my own kids to go over there as often. One time my eldest told me that their four year old found a fish hook and was stabbing it into the couch during a playdate. WTF?
It might change things, since you two are friends, and thus you are more in a position to talk to her than I am to my neighbor. But there's not that much you can do. None of these things are CPS worthy, at least I don't think so. I would try to be subtle about it and discuss things in terms of what you have learned about safety and your own baby. Like, for instance, you could say, "OMG, my baby almost rolled off the changing table yesterday when I turned my back. It was really scary. I've learned to always put my hand on him/her when I turn around to grab something."
I think letting the 16 month old alone in the backyard is concerning, especially if there are grills and other things around. That would concern me the most. Also, if they are that neglectful, how are baths? Does she allow the 16 month old to be in the bath unattended? I would be extremely concerned about that. I know this sounds terrible, but if you ever see tragic stories about babies or kids dying, you could also mention that offhand - like "did you hear about that poor little baby who drowned in the tub last week?" This would be less confrontational yet still wake her up to the reality that bad things can happen, especially when very young children are unattended.
On the car seat issue, I have real problems with how people put their car seats on shopping carts. It is completely dangerous to put your seat dangling on the top. Kids have died that way. The seat should be in the cart itself, but most people don't do this. I often want to say something, but I know it's not my place.