I think you're at the point where you really should see a lawyer. Continue documenting it. A L will fight for you and bring it up to the judge in a businesslike, matter-of-fact way.
During my temporary orders court appointment STBX wanted to collect back support retroactive January. I had a perfect relation of what I paid of her house (mortgage, bills and such), the source of the funds (to prove it was my separate property), and the half and half portions, which proved that she actually owed me money.
A L can help you arrange to get that paid back, either "in lieu of support" or as part of the 50/50 settlement.
Don't be nervous, he definitely owes half of those taxes. And now he owes you. Don't file together again, and make sure during your D negotiations you agree on who files the deduction for DS, and make him agree in writing to pay half medical expenses.
Now for my next trick... I need to find out a way to make it more efficient for my L to collect so it takes less attorney's fees. I figure if I have my L contact hers, so it costs her L time as well, she'll start actually paying back regularly.
It's so sad and so stupid for us, to see the waste in L representation, but make a cold calculation of how much it costs you in your time and aggravation, plus what you recover, and the L's hourly rate. You may find you come out ahead.
I'm so sorry you're having to go through this.