Oh well, can’t help her then! Except to say I agree with what the righteous Sage Rat says.
OP, have you ever considered some sort of assertiveness training? Because it sounds like, until you learn how to calmly stand up to these idiots, you’re always going to be the doormat. I understand your retisence - I was always pretty shy myself and really had to struggle to combat that side of me. Some sort of professional training may help.
My girlfriend manages a concession in one of London’s biggest, most arrogant department stores, and the staff there sound exactly like the people you have to work with. Evil boss that everyone’s scared of, jumped up young floor managers who pass the buck and bully the concessions. Her concession had previously gone through 5 grown managers who couldn’t cope. Her first few weeks were a nightmare - she was constantly harangued about sales figures, her staff, her stock, you name it. Even her Managing Director couldn’t cope with them.
However, my girlfriend is a very confident person and started to fight back. She stopped taking their criticisms and started throwing stuff back at them - “Can you help me with this? Why not? How do you expect me to increase sales if you won’t let me do x? I’m going to do Y unless you give me a damned good reason why I can’t.” When they don’t respond to her requests, she copies the evil boss in on her emails. They don’t know what’s hit them, are now being nice to her staff and are frankly looking scared of her. And her figures are going up. BUT she is assertive. You need to learn some of that.