How have fuel consumption and emissions per passenger/kilometre declined since the first jet airliners?
Don't know where you can find those details (ICAO or some UN organization maybe?) but you could ballpark it by looking at the specific fuel consumption of a 1950s airliner engine and a modern one.  

The JT3/J57 turbojet used on the 707 in the late 50s had a specific fuel consumption of .909 lb/(h lbf) at cruise, and the GE90 used on the 777 is .545.  This means the fuel burn per unit of thrust is about 40% lower on the GE 90 than the JT3. Since a jet engine is just a fancy oil furnace, without elaborate systems to reduce specific kinds of emissions like car engines, the SFC itself should be a good  proxy for overall emissions.

Being conservative by assuming the thrust per pax is roughly the same for modern aircraft vs old ones (I'm sure it's lower), you could use that comparison as representative of the industry and say emissions are at least 40% lower per seat mile now than in the 50s.