A CA not located in your state most certainly CAN sue you. They have to sue you in your county however. They simply get a lawyer in your area. (Though some purposely try to sue you outside your county, but that is easily fixed if you catch it.)
If they refuse to validate then there is a few ways you can go. You can ignore them, or you can go after them legally. If they can't prove you owe the debt then they are reporting information falsely on your credit report and they must remove it. If they sue you without validating, you demand the documentation in discovery and make mention that you requested validation and they did not provide it. You may also be able to countersue them for violations depending upon how they handled it.
They DO NOT have to work with you at all. They do not have to agree to any payment arrangements. Now, there is some states that has laws if you send them a payment and they refuse it, then they have just lost the right to collect upon the debt. I don't know which states do this, you will have to read your state laws on the matter.
The reason they are so mean is they are banking on your not being aware of your rights and the laws they must abide by. They try to force and cajole a payment out of you with strong arm tactics, shame and even threats. They are hoping you cave in and pay them without a fuss.
You need to send a cease communication letter to them certified mail, informing them that you are not allowed to recieve calls at your place of employment. If they call you after that, then that is a violation and you can sue them.
Yes. You can claim your debt in bankruptcy and there is not much they can do about it. Once it is in bankruptcy they are not allowed to attempt to collect from you. I am not sure if they can report the debt..I think they can if they list it as being discharged in bankruptcy, but I am not sure on this.