Fulbright could pull the whole von Karma "I claim the stab on my hand has nothing to do with UR-1," so just having a stab wound in the same place would just be treated as circumstantial, not conclusive. He shot the rock into space because the plan was for the rocket to be destroyed in space and never return to Earth. If the Phantom was posing as a space center member, there's a decent chance they have some data on his DNA. It would just be under an alias. At the very least, the robots have facial data on him. I imagine he didn't kill Athena and clean up the scene of the crime because he panicked due to his own blood being spilled on the rock and he wanted to get out as quickly as possible before anyone else showed up. He wasn't trying to cover up the fact that Metis had been killed anyway. Plus, Athena couldn't have identified him since he was wearing a mask.
Now Fulbright being dead all along was a twist for the sake of a twist, but the other things do at least have plausible explanations. ---
