Re: Why does plain water rust nails in closed jars more than saltwater(2tsp/c)?

The corrosion of iron is well-studied, with many books published - so
(university) library!  Rusting is subject to considerable experiental
manipulation.  Were all variables in your comparision rigrously identical
except for the presence of salt vs fresh water?  Rusting may require oxygen
in air.  Having the nails fully immersed versus partially immersed can make
a big difference.  The amount of air in the water also makes a difference,
boiled out versus shaken up with air.  Were all the nails identical?
Alloys, physical treatment during manufacturing, and surface treatments 
vary- even to scratches!
Do either 7-UP or vinegar dissolve rust?  Will dissolved organics adsorb on
the metal surface to protect it?  Consider oleoyl sarcosine used to protect
iron against attack in hydrochloric acid pickling baths (to remove oxide 
scale).
There are no simple answers, which is one reason why experimental chemistry
is interesting.