I’ve got experience in the hitting people, and getting hit department. Does that count?
The maximum damage you described would be difficult to achieve, anyone powerful enough to break an arm like that would be able to knock him out barefisted. Shoulders are routinely used to absorb hits in combat sports, especially boxing…well, more deflect, but they do get hit, and apart from thighs they have the best protected bones.
The numbing thing, sure… sufficient hit to the brachial plexus will render the entire arm numb instantly… and probably be a pain in the neck too (pun intended). That’s usually a fight ender, the arm is useless for a while.
Enduring numbness, again, it’s possible; A pinched nerve will cause numbness, tingling, loss of motor control… it’s usually due to compression of the nerve from swelling in the surrounding tissue. Damage to the actual nerve is also possible, but I got the impression you’re looking to have it clear in a week or so? Damage to the nerve itself tends to take longer to heal, that is if it heals at all.
As for screaming… that’s sort of individual. If there isn’t a jagged piece of broken bone poking under the skin, I would guess no. Let’s put it this way, I once jumped out of a helicopter… and landed badly (“knee… meet rock”-badly). Snapped the ACL, fractured the patella, crushed the prepatellar bursa, and tore the MCL in half. I didn’t scream when someone grabbed the knee to check what was wrong with it (and definitely not gently)… I grayed out, and seriously considered shooting him… but no screaming.
If I might make a suggestion? If you’re not too set on the shoulder… you should take a look at rib injuries. A fractured rib would work better. There’s even scalability of injury, from a single rib injury, which is endurable and will heal quickly all the way to multiple lung penetrating broken ribs causing internal bleeding that requires hospitalization. Taping up broken ribs hurts, and can be done at the home in emergencies (or just to show masculine bravado). Also ribs would make for a natural target for brass knuckles, they’d give the attacker just that extra hardness to ensure damage.
…just saying, is all.