If a party receives an inheritance and keeps the money or property solely in their name, the other spouse, in the case of divorce will only be entitled to the appreciation of the asset. I would caution you that the inheritance should never be commingled with any other marital asset and should never be placed in joint names or given to your spouse. If you do so, the asset will be considered a gift to the marital estate. How it will be divided will depend upon the county which is handling the divorce.
You do not need an attorney to file for custody. I think you should reconsider why you are even filing for custody when you have it by default. Even if it written in an Order when he is going to have your daughter! a court is not going to force him to take her. I would recommend that you file for child support so that you have it wage attached by his employer and that you know that you are going to receive it.
To serve via public notice, a party has to file a Petition for Service by Publication. However, service by publication is generally reserved for those cases where the party cannot be located. I would suggest that you retain the services of a constable as it appears that you know where your husband lives.
She cannot collect child or spousal support if there is no Order directing you to pay. She is just blowing smoke. In fact, it may be that you may qualify from spousal support her. Feel free to contact me to further discuss your rights.
If your goal is to get primary or shared.custody, I would not leave your son. I would take him with you and immediately file for custody and divorce and try to work put a reasonable custody schedule in writing and filed with the court. Although neither of you have a greater right to your son than the other leaving your son with him is not a good idea. Also if you are concerned with his mental state would it be in your son's best interest if you left him with his father after you left. Your MIL...
You can always represent yourself, but that may not be the way to proceed. You should have a consultation with a family law attorney to advise you as to whether you are able to proceed pro se (without an attorney) based upon the the facts of your case.
The documents to bring are any documents that demonstrate that you and your husband acted as a married couple during the time period in which he is saying you were separated. These include Birthday cards, anniversary cards, joint bank statements, joint credit cards purchases, and pictures of you and he together at functions or trips. I am not sure what county your matter is in but you should be prepared to offer testimony and exhibits. You may very well want to consult with an attorney before...
Depending upon who your Wife's employer is, they may not let her take you off until there is a change of life, I.e. divorce, death, or she can show proof that you have your own insurance. The best way to guarantee that she maintains you on her insurance is to file for support and have that part of the Order.