Your attorney sounds like they have a good hold of this for you.
Your documentation will help you with his allegations of you interfering with visitation and you can't be held in contempt for the child support issue, you weren't court ordered to do anything in regard to that.
The court may find / calculate he should have a credit but if that happens, it will very likely just order a reduction of xx amount for so long till it is balanced out....the courts don't typically order no child support like he wants. and that reduction would likely be handled the way arrears is handled, a small amount each month (if the court even finds he has a credit).
If this never went through a child support agency, then maybe a good suggestion would be to get that ordered so they (agency) can accurately track his payments. The employer shouldn't have had to do that, they aren't responsible for calculating when he paid off his arrears and medical payments, so you may want to ask your attorney to have child support sent through your state agency.