Aurelius

You figuired right.
Can't advise you on the second question because I don't know what the nursery was using. I think it's safe to assume that overfertilizing isn't your problem though. Most nurseries use a time/temperature/pressure release fertilizer in their soil. They look like blue/green/yellow uniform balls peppered through-out the soil.
As for the airlayer, wait until the first new growth has hardened off.
Early in the spring the flow of energy is flowing upward. Making cuts will just bleed and possibly kill the plant. Later the flow is going circular. As the flow goes back down the trunk it hits the bark ring that you made. With no-where to travel downward anymore it kind of decides to make roots.