One can't really say how long it will take as everyone is different. Would be great if you could attach an image from your latest x-rays though.

You will have disuse atrophy from the cast however it is more likely that the brain is inhibiting the muscles from working. This is because some of you biceps group attach across the fracture and as they have been immobile since the fracture they are not sure what damage (or not) they might cause if they were to contract. The fact you can move with assistance is great. Perhaps try the same movement lying on your side and or in the pool.

You are months away from being anywhere near normal and right now your bones are only just healed. They will take 12 months to fully ossify. You might get your friend and or PT to gentle loosen off your neck muscles as you would have sustained a neck injury at the same time during the fall. Of course pain and focus went to the obvious fracture so you wouldn't have considered the next at the time.

For future ref, if a plaster slab or full cast is really loose, have them cut it off and re-do it. This is common place in Australia and should be elsewhere. Some people are just plain lazy