Do you know if the Jag is pure coastal or if it might have some other carpet in it? If it's just coastal when younger they will climb a lot but as adults they tend to just stay on the ground. Not that it won't still be a great display snake. you just wont really need a tall tank with lost of branches in it. Jungles will use perches more through adulthood.
I've never had a wobbler but from the people I know that have them they've never gone away. A breeder telling you they never saw it wobble could easily be because breeders usually have such big collections they tend to have less time for each individual snake. Which is understandable. And with your snake so young they would have barely had it in their possession themselves to potentially notice it. Or he could have just been lying.. really they shouldn't start selling the snake until they know for sure though.