I am not admitted in Michigan.  Threaten you with what?  Actual proper service of divorce papers?  I wouldn't sign a thing.  It is when they are served that matters here.  

And for the portion regarding your son, if you are contesting it it is doubtful that the court would give her sole custody except in extreme circumstances.  The court uses "the best interests of the child" as criteria for determining custody issues.

Sounds like your wife is just angry.  If you think that she is serious get yourself a lawyer.   

There are too many details missing in your question to give an answer.  However, joint custody is the prefererence In Michigan.  Also, from the facts you supply it doesn't sound like enough time has passed for a court to have ruled on the facts of your case.  I am admitted to practice in Michigan if you would like to discuss your case further with me please  feel free to email me at jrothstein05@gmail.com