First off, you have to move parts even a bit from the first frame position to set them to keyframes. Unless you set those parts so, they are overridden by any animations, so that they move because you can't give any priority to non keyframe's parts.
Second, by the animation of the same priority, one which was performed later overcomes another which was performed previously. And the max of priority is 4. As you know, there are a couple of built-in animations have priority 4.
From: Seagel Neville
Even though you create your animation with a max priority, when other max priority animations are played afterward, it is overridden by them.
So I suggested that you had to play your broken arm animation after such max priority ones to overtake them.
By that script, it playes your animation every 0.5 secs. So even though any max priority animation is about to play, you animaton is always performed just after it again and again.
But it seemed not to work for you. I suspect that you failed at the first step.
1. You have to create 2 frames animation. The first one is the defalut T-Pose and the second one is the posture you like.
2. To create the posture you like, you have to move Collar, Shoulder, Forearm, and hand. Even though you want to keep them the default positon, you have to rotate them even 1 degree at the second frame.
3. Set the priority 4 and Loop on the uploading window. Then, upload it.