your given measurements are identical to my own and my ideal fit typically has me on a 50 c-c with a 52.0-52.5 tt and 100 mm stem & setback seatpost. I can get by pretty reasonably on most 49's using a slightly longer stem ,setback seatpost and more stack height/upturned stem and could push a 51 in a pinch with some creative tweaking but no way would a 53 st and 54.5 tt ever be an acceptable fit for me.
unless you have an unusually disproportionate torso to leg length, I'd say that the 53 is probably too big and that you'd ultimately be happier riding the 50. I'd be inclined to find a different shop that'll work with you.