Metatarsal fractures generally heal well, even without surgery. This is most likely why your doctor said she was originally going to cast it. It's always important to get a few views of the foot when evaluating broken bones, because not all views will show the displacement. On the AP view (top to bottom view) there is a little displacement, mostly shortening. On the MO view (the 45 degree angle view) the shortening is quite evident. This is probably the reason the doctor changed her mind. This picture will give you a little bit of an idea of how the 2nd metatarsal should look (keep in mind that the length in this image will appear exaggerated on this view - the MO view).
Will your broken bone heal without surgery? Probably. But, when the 2nd metatarsal is shortened, it can cause problems down the road. The second metatarsal should be longer than the 1st and 3rd - when it is not, many people will develop pain under the 3rd (and/or 4th) metatarsal head, this can also cause the big toe to take up more than it's share of the weight, causing big toe joint problems and some people have a floating 2nd toe (the 2nd toe doesn't touch the ground). These are all just possibilities.
Yes, it is borderline. Some doctors would probably recommend surgery for the above reasons and others would put you in a cast. It's always good to get a second opinion.