Q: Do you shoulder the 15" barrels like you would a 26" barrel or do you use it like a pistol?
A: Under Federal law, it's illegal to use a shoulder stock with a barrel less than 16 inches in length so you can only use them legally as a pistol.
Q: What are the advantages and disadvantages of using these shorter barrels?
A: The advantage is to use them as pistol barrels.
Q: Do you lose any range by using the shorter barrels?
A: Since you lose velocity with a shorter barrel, you also loose range. Also, though a pistol barrel doesn't have any decreased inherent accuracy, a pistol is harder to shoot accurately so unless you are skilled and shooting from a very solid rest, accuracy will not be as good at long range with a pistol as with a rifle.
You didn't ask, but there is also State law to consider. Some states require handgun registration. In such a case, using your Encore frame with a short barrel as a pistol would be illegal under State law, unless it was registered appropriately, but not under Federal law.