The third wire is the power wire for the pump. If this sending unit is for external pumps then that won't be an issue.Yes, I looked at my spare sending unit and there is only 1 wire to the float. What was the 2nd wire used for? A low fuel level light maybe?
Having the three prong connector on the sending unit would be nice as it would require no wiring (just leave the third prong empty). If not, I'm sure buyers can make their own connector.
I'm not sure how to answer the vent question because I don't know its importance. However, how difficult could it be to add one more tube from the sender?