If they're controlling the metal via TK, how would that not be them continuously applying force? Unless you mean in my analogy, where I specifically mention the Bowling Balls have two constant forces acting on them ("Blowing"). Either way, I don't see your contention here. The initial force is so high because the attack had distance to accelerate. The fact that it halts when clashing with someone in and of itself shows it lost all of its own momentum. What's causing it to continuously clash is the TK constantly acting on it. But the TK itself only made the object move that fast via accelerating a certain distance, yeah? Distance that it can no longer accelerate through because another force of equal force is pressing against it.
I don't know the context of the feat, but this would help. Did the metal in question get its force via accelerating? If it did, then I don't see why they'd scale to the full yield when clashing with it because it's speed would've momentarily dipped, and it would no longer have the ability to accelerate if it's clashing with two other people. Because it's be "pinned" into a standstill. It'd be one thing if they were resisting the TK by itself with no metal object, because the large metal object weighs a lot, and thus take time (acceleration) to build up to the full yield of the TK. Whereas the TK when only acting on the person can exert the full force without being slowed down.
Again, I don't really know the context of the feat, so I could be missing details that change if it's valid or not.
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