Not so sure that 16" wheels and tires are lighter than 18" wheels and tires. But the 18"s definitely get lower mpg, certainly due to the increased width, and possibly because of lower compliance with narrow sidewalls. 16"s have other advantages such as lower replacement cost, lower road noise, and lower chance of rim damage from road hazards. Seems like an irrational choice to get 18's other than no other choice for a model trim you want. That said, the 18's due to their width and lack of sidewall compliance, will corner faster, brake shorter, and "feel" better.
I seriously doubt that the base FE and the base EX (without options) are more than a few pounds different. Most of any mileage difference will come from the roof rails on the EX, which are removable. Part of the issue with the lower mpg rating of the EX/LX comes from technical details of EPA ratings per weight - you can have a decrease with 1 pound of extra weight between two weight tiers. The larger mpg decrease of the Touring models is largely related to the wheels. However, they are heavier, by about one passenger. This is directly reflected in slower 0-60 times as tested by Car and Driver. And that weight does have a mpg impact, depending on how heavy your foot is, and the hills in your normal driving.