I like is well enough, but I too would probably go for a more traditional color and style (dark blue or black with hand warmer pockets) for a pea coat. I have never owned a Marc Jacobs coat, so I can't comment on them.
I'm almost certain the coat is real. i agree that a darker, more traditional color is better, however, is the lighter color too weird? I feel like the grey is still dark enough so that the coat isnt too unusual.
I like the light grey shown there. If it was my size, I'd be bidding on it too! If you like it, and it will fit you, get it. I don't necessary agree that anything remotely similar to a peacoat-style cut must be black and an official USN peacoat (or try darn hard to look just like it).