OK, I try to summarise (while the kids are still asleep):
King used an existing brand and existing car models and made up his own version of a car. Just as he could have made up a La Z Boy Recliner with a slumber air mattress option - possible, but never produced and on sale like this.
Possible reasons: He didn't know any better (very unlikely), it better fits the story (innocent rich single woman with a SL 500 plus more power to use the car as a dangerous weapon), King doesn't want to get in trouble with Daimler (hmm? it's not the car who killed but the driver, and at the end of the story the car is fast enough to get the trio to the next setting and to prevent the next attack; and Holly is quite happy with her baby blue MB).
The difference between a fictitious La Z Boy and Mercedes: The recliner is not mentioned in the title of the novel, it's not used as a weapon, it's not hiding the 'key question' the cops keeps so busy and on the wrong track for a long time, it's not used as a weapon to find the perk/p, it's not used to distract the perp/k, it doesn't help to prevent the next attack and catch the perp/k, and it doesn't accompany Holly in her "new" life. Besides, I've never heard about the La Z Boy brand before whereas the brand Mercedes.... and the brand SL... They probably evoke quite distinct images for many readers, some of them end up in this forum
I don't know about Jan's agenda, but mine is quite simple: get ideas about King's reasons, get ideas about how it effects the story, and the readers (I'm doing research on facts in fiction, especially brand images in fiction, especially cars, especially Mercedes). That's why I keep questioning
But thank you so far!