Man that is so horrible. Not all places are like that. Unfortunately it tends to be smaller places that are more trustworthy. I suggest that at very least you may need a good immigration attorney.
Family companies are the worst in my first- and second-hand experience. Blood runs thicker than water, and outsiders are often reminded of that fact come promotion/major decision time
If you didn’t experience it yourself it’s solely due to a mix of luck, never being at places that struggled and being hard to replace, that’s it.
Offer letters never provided much of a legal protection. We just treated them as if they did.
Amazon et al wouldn't rescind them by their thousands this year if that created a strong legal case against them.