Admiral Marcus wanted to start a war. If he sent the torpedoes to Qo'noS, the Klingons might not know who sent them. However, if he sent a ship - which was sabotaged in such a way that it would be stranded near or in Klingon space, and that captain then followed orders, which was to send 72 armed torpedoes onto Qo'noS, the Klingons would quickly locate the vessel, take out the vessel, and blame the Federation for the attack and go to war.
As for world building, I don't have problems with what has been established before. I do have a problem with a Klingon Empire that has a sensor net so porous that a lone Federation ship can sneak into or near Klingon space which can either torpedo their homeworld or send a K'normian trade ship to the surface. (I had a struggle to understand the geography of the action. Where exactly was the Enterprise - was the ship in the Neutral Zone or in Klingon space? And, how far from the Neutral Zone was the Klingon homeworld?)
The novel does a better job on the science aspect of the story. In the novel, I learned that Khan did a series of leaps to get to the Klingon homeworld, and that McCoy had to inject Kirk with medicines so that his body would accept the new blood.