"Buckwheat" was a character on an old children's show in the US called "Our Gang" or "The Little Rascals." Do an image search for "Buckwheat Little Rascals" and you will understand the reference.

That is very hard to explain. it is a common type of expression used to indicate surprise. Sort of similar to "Well, I'll be a monkey's uncle!" which you might have heard of. It is a common type of expression in the US South, I think.