A) It is nowhere near a 53, more like a 63. There are several sites that provide Schwinn serial number decoding guidance. It is pretty hard to get this wrong.
B) I do not think that seat is original. At least I have not seen it used on this model before.
C) I would put a value of $75 to $100 on it. Some newby, less jaded than most of us, might go up to $150.