It depends upon the guarantees or representations made to you by seller. Read your contract.

Do I have to file income tax in Michigan in this situation?

An accountant or tax preparer can review your income and provide a response.

Would like advice if I should seek legal counsel or law is clear & just give ex 2015 income amt & expect no further child sup?

Your child must continue with post secondary education for child support to continue until 21.

To get a condo tax abatement in NYC, does the residence have to be your primary one?

In other states yes. I encourage you to speak with a NY attorney to confirm.

Sorry that sounds like a difficult case to prove. Sit and discuss with someone local. Perhaps there are facts that you just didn't share.

How can I change my child's last name?

It's a petition before your local circuit court.

Why would I have to be responsible for a mortgage that my name is not on and house was left to me in a will

I can't imagine you would be unless you signed a promise to pay. Your mothers estate is liable.

Is it legal for me to keep records pending arbitration? Any advice? I feel like I have gone through the rabbit hole here!

Sounds like a question for an accountant or someone familiar with their rules of ethics.

Should you open an estate even though a beneficiary threatens to litigate?

Arguably if an estate is opened it provides all a forum to litigate any differences.

No that is self help disallowed by law.