Have Tax Questions? Ask a Tax Expert for Answers ASAP
All that you have said is correct. You cannot take your daughter as a dependent unless her mother signs Form 8332. You cannot take the medical insurance as a deduction since it was already done pre-tax.
I wish there were a better solution for you, but you are pretty much stuck owing the money you calculated. If you cannot pay it in full you will need to set up an installment agrement with IRS to start making payments - it is possible that your child support payments can be lowered as a result of the tax debt but that would probably mean going to court. It is possible an attorney or court order might be able to get your child's mother to sign an 8332 for now and in the future but will certainly not go back to previous years.