Hi, I like your post but I think there are a few issues with your proposals. While para's validhunting rules could maybe be made clearer they're generally in a good place right now. There are a couple of problems with allowing the use of lethal force against antagonists as a non-security crew member:
1. You're just some random NanoTrasen employee, for most characters and situations it makes very little sense that you would be prepared to lay down your life to protect a coworker instead of fleeing and calling for the team of jackbooted thugs that do this for a living like you're trained to do. Additionally, if you kill someone without being able to provably demonstrate that your life or your coworker's life was in serious danger it's clearly murder, and demonstrating this beyond reasonable doubt is difficult to do.
2. There is a fundamental problem with being able to use lethal force as a crew member for the defence of someone else - if any coworker is attacked you could usually just lethal the antagonist since if you try to interfere with an antagonist where you shouldn't, they're just going to attempt to execute you and continue their task, as well they should. This provides justification for you to attempt to kill them and would lead to antagonist gameplay feeling a lot more crew vs antagonist instead of security vs antagonist, in my opinion.
Overall I think your suggestions are reasonable and well considered, but I feel that the current system of lethal force only to save your own life and only when there's no other option provides the healthiest gameplay for both crew members and antagonists. The antagonists aren't swamped by crew members rushing to stop them in defence of their friends since the crew members understand the high risk of unceremonious death and the crew members will be treated more easily by antagonists when they interfere less, and escape with minimal injuries more often. This also allows for stronger rules against collateral damage with antags, since bystanders represent less of a threat to the antagonist when they're not going to attempt to murder them.