Chang Feng, I shall attempt to answer your three questions:
From my understanding, I don't believe the concentration of product effects the rate of reaction of an enzyme. Maybe in a confined area, the number of products may increase overtime, thus limiting the space through which enzymes+substrates can travel. Therefore, there are less enzyme-substrate collisions in a given time frame, hence slowing down the rate of reaction. Or maybe even, as the product fills up the area of enzymes, the space is limited for which substrates can come in. Hence, less and less enzymes can catalyse reactions as time goes on. Though I've personally never come across this given situation.
As for enzymes reducing the activation enegy: An enzyme’s selective three dimensional shape, the active site, is complimentary to the substrate it binds to. When in the active site, the chemical bonds are closer, therefore allowing the chemical bonds to weaken and change with less energy. An enzyme therefore lowers the entropy of substrate, freeing them from translational and rotational movements.
Coenzymes are organic molecules that are required by certain enzymes to carry out catalysis. They bind to the active site of the enzyme and participate in catalysis but are not considered substrates of the reaction. Coenzymes often function as intermediate carriers of electrons, specific atoms or functional groups that are transfered in the overall reaction. An example of this would be the role of NAD in the transfer of electrons in certain coupled oxidation reduction reactions.
Cofactors are often classified as inorganic substances that are required for, or increase the rate of, catalysis. This binds to the active side, and still allows for the complimentary pairing of the specific substrate, but provides the necessary inputs for the reaction to take place.