I agree that this does not sound like a pro bono case. Without knowing more facts, Judges generally enforce court orders (like your divorce Judgment) based on what is written in the agreement. If it says she needs to indemnify you for utilities, then the court the court would likely make her pay. You haven't stated what her argument for no paying would be. Having a father who is a high profile attorney probably won't help her. Did her father represent her in the divorce?
You need to hire your own lawyer. You can't afford to not have a lawyer. His lawyer cannot ethically represent both of you. You have a long marriage and issues of maintenance (alimony), asset and debt division I am sure exist. The court can award you interim attorney fees if you can prove you do not have an ability to pay your fees and he has the ability. Normally, if one of you can afford to hire an attorney, then both of you can.
You should probably be thinking of divorce and not a legal separation. What are your reasons for seeking a separation and not divorce? Do you have an income? You would need income in order to be ordered to pay support. I would need more facts.
It is really worth it to hire an attorney to help you out. Normally you need to file a petition for temporary child support. Do you already have an order stating you are the temporary custodian of the child(ren)?
It sounds like you have a dispute with your eldest child over the custody of their sibling (your child). You will need to take your case to the probate court or whatever court gave guardianship or custody to your eldest child. Contact a family law attorney.
You will need to go back to court to force him to turnover the documents he agreed to in the MSA. The request is callled a Petition for Rule to Show Cause. You should have an attorney help you. The court can order him to pay some or all of your attorney fees for having to go back to court. Sometimes just the threat or a letter from an attorney can get him to comply.