I have lived in Georgia. Getting milk home asap is the #1 priority of groceries. Even here, unless it is very cold outside, i get it home asap. It tastes better imo. However, milk gets condensation on it if it is cold and the day is hot. This is how it applies to SW.
The milk jugs ought to have been wet, therefore getting her wet. But, remember, she was cold. So, probably the milk was no longer cold in that photo, so there was no condensation to get her (and her light colored clothes) wet. Especially if you go with CB saying SW wasn't sleeping like that until they were close to home.
People are warmer than milk out of a cooler. H had the milk on his lap because he didn't want to clean it up. Idk. I wouldn't sit holding two gallons of milk on my lap. But, apparently he did although he was confused about how he got out of the car. He finally decided he put the milk on the other side of SW.
It's just an odd story. Like maybe it somehow didn't happen like that. But, obviously, they have the milk and H got home, so... It happened somehow. Just like SW being awake or asleep at that point varies. I get that maybe they don't remember and are sort of filling in the blanks sometimes.
Like maybe the idea that SW was asleep on the milk at A's house comes exclusively from the existence of that photo which, by what CB says, and the scenery happened later.
This is the last known photo of SW. So, this is why it is being analyzed. She was with her mother and grandmother then. It's sad.