There is nothing you can do to prevent the county attorney from charging. In the event there is a charge, you should obtain an attorney knowledgeable in the area of Criminal Defense. Do not try to do this yourself.
I agree with everything Mr. Standke said and would like to add some thoughts. IF your court order for visits with your child is in MN then you may be able to go to the Judge and get a special emergency Order giving you temporary custody IF your child shows up here this summer and IF she tells you the things that she said last summer. IF you get a temporary Order then that would force Mom to address the issues.
IF you have a visitation order from TX AND your child shows up here this...
Depending on the rules in your jurisdiction, it may be possible to get a special court order after filing for divorce to have your spouse served by alternate means. Sometimes it is publication in a legal newspaper serving your area. Or he could be served by mailing to his last known address if that is permitted in your state.
Your son can be adopted by your fiance through a stepparent adoption. It is a fairly easy adoption to do based upon your facts. Sometimes the lawyer will have to do a father's registry search to complete what needs to be done. The proposed father may have to undergo a background check and sometimes there is an in home study that is usually waived if the child has been living with you both. A family lawyer who does stepparent adoptions in your area wouild be the person to contact.
I am unclear about what your exact questions are. You should consult with a divorce attorney immediately in your area and find out what your rights are. Some attorneys will offer a no cost consult. Go now.