I work in both areas of art production.
Commercial art is directed by marketing decisions and fine art is directed by emotional and intellectual exploration. Often more than not commercial art is a group effort that expresses the producers desire to be accepted and purchased Commercial art helps producers sell product that are not yet known or that need further exposure to be commercially viable.
Fine Art production is a discovery process that has no set of prescribed intentions other than being responsive to the interaction of mind and body. Art that seeks to extend the envelope of what Art can be is often has a small audience because it can not be labeled as there is nothing like it to compare.
I don't agree with the division that commercial art is production and fine art is invention. Many artists work in both fields. You are correct that a commercial artist must know production but they also use invention. You are more correct if you were describing a beginner in the field of commercial art who have to follow direction of the more experience artist in the company. Innovation and invention must exist to succeed. Advertising, textile design for printed fabrics have a lot of invention. In the case of fine arts, the artists had better learn enough about production to be able to show their work. It is necessary now to deal with newspaper companies to submit digitals with high enough dpi so that they can be printed.
The use of the computer has become necessary to market art work. It must be understood by the fine artist of what will print well and what will kill the art, even if they hire someone to do this for them. True some still stay in their caves and never market. I think what you mean by invention is ideas and originality. I have to say that the best of both commercial and fine artists exists when they show or use invention. So, there is a lot of invention and production knowledge in good commercial work, and a lot of invention and production knowledge in good fine art. I would say that the difference between fine art and commercial art is who or what their target or goal is to please, whether it be themselves or certain groups in the market or the world.
And from Charlie D:
Commercial art is art produced for a customer whose needs and wishes must be satisfied. This does not mean that they have to be followed. The artist has flexibility in how this is done but the end user is the judge of the product. Fine art is the work that satisfies the needs and wishes for the artist. Its production is completely up to the artist.