Gentlemen, STOP your engines!!
This has to be the easiest question ever posed on Tidefans.
There are two terms in question, capstone and keystone. They are related.
Keystone is simple. When building an arch, either Romanesque or Gothic, the sides are raised, with appropriate scaffolding, then the arch begins to take shape. When the masons get to the point where one stone in the center holds the arch in place, that stone is the "keystone." Look at a map of the United States in its early days and look closely at Pennsylvania. You have one part of the country arching to the right (ending with Maine) and one part arching to the left (ending in Florida). Get it? Doesn't that look like a "keystone?" From a philosophical standpoint, the keystone is the entity that holds things together and makes them work.
Capstone and keystone are almost synonymous. Sometimes the capstone will be the keystone, but in other cases the capstone might be the crowning stone that "tops off" the structure.
Consider education in Alabama. Where does the University fit? EUREKA! There it is as both the keystone and capstone of education in the state. Q.E.D.
By the way, I had this explained to me in 1967 by George Toeffel, one of the all time great chemistry professors at the University. When I started teaching there it was my great honor to work on the same faculty with him. He had a wonderful "feel" for all things Alabama.
People like Toeffel are the ones that made the University the Capstone/Keystone of education in the state.