She enter the US legally so she doesn't have to return to her country for consular processing, she can adjust status here. As soon as she is married to your son, your son should file I-130 and I-485 concurrently. If she wants to work or travel prior to receiving her green card she must file EAD and advanced parole at the same time or soon after. If everything is fine, she will be getting her green card in 3-4 months after the application. If your son is ever unsure about the process, he should...
There are no forms, derivative citizenship is a matter of law. You can derive US citizenship if your mom is a US citizen, you were under 18 years old, you were a green card holder, and you reside in the U.S.
First, I am sorry to hear about your loss and the grief this accident has caused you. I agree with my colleagues, you should consult with a personal injury attorney. You shouldn't try to resolve this one your own.
Most personsl injury attorneys will work on contingent fee based, that is you won't owe attorney fees unless the attorney recovers money on your behalf. You should definitely contact an attorney's office immediately.
I agree, the fact about your baby is an important testament about your hardship. By the way, I hope you are getting an assistance from an immigration attorney in filing you I-601 waiver, you don't want to jeopardize it by trying to do it on your own. If you have retained an immigration attorney, your attorney should give you inputs. Best of luck.