When calculating the wing area, load, and CG of a mid wing plane, do you count the area within the fuselage width?

Yes.

HTH - Steve

Yes and no, it won't affect your real outcome to much, but it depends on the fuselage shape, size, ect. Many do, I don't, unless the plane is a lifting body type. -Happier with the percieved performance.

Jetts

In full scale aircraft design, the wing reference area is calculated from tip to tip, including the portion that intersects with the fuselage(s) and/or boom(s), etc.

It is a matter of convention.

Using design tools like the Datcom which is a set of tools and charts that let you use a sliderule (bamboo calculator, no batteries) to design an airplane, the authors had to make a decision. By including the fuselage they could develope a set of tools for the wing. Then modifications to the pure wing characteristics due to the fuselage were another set of charts.

It is like any science, engineering, rock throwing or pie baking thing. It helps if everyone is talking the same language.