Its all about the custodial parent
Let me begin by saying; your options are limited.
For the most part, the state will always side with the custodial parent when money is involved. What most people don't know is that these agencies are agressive because they receive federal funding based on the success of their collection rate.
These agencies follow strict guidelines as written in Family Law and will seek the maximum amount allowable from the non custodial parent.
What they tell you is the money is "In the interests of the child", but they fail to validate how the money is spent. As I was told "it's none of my business" how my ex uses the money so long as the child's needs are met. Again, they have a hidden agenda to maintain their funding.
To answer your question; these agencies will seek "all" money earned and only exclude federal, state, and social security taxes. Other factors will be any medical support being deducted and union dues.
A word of advice though; don't assume your husband can temporarily increase his tax deductions to lesson his net income. The states will compare his income against tax tables and determine the amount he should be paying.
In may case, I put a substantial amount away each month in my 401K. Guess what? That's still considered income and they will count it as such.
As far as the custody issue, as long as she maintains custody, she receives Child Support. You could always seek custody, but it will get costly. From what I know, the only way a father will win is if the mother is determined to be unfit. Usually that doesnt happen, unless she displays a "Britney Spears" pattern of behavior.
What I did find out was that a custodial parent's sexual bahavior may be used to determine if she is fit. If you can prove that she has multiple sex partners and they frequent her home, then a judge may decide the environment is not conducive to a heathly environment for the child.
Your last option may be in the form of joint custody where both parents share equal responsibility in the child. Not knowing how far the parties are apart from each other, the child may be able to live in two places so long as nothing else such as school is interrupted.
The one thing you have going is that the child spends most of his time with you. If you can prove that, it may work in your favor.
When it comes to visitation, both parties are bound by the existing order. If at any time she fails to follow this order, you can take her to court and potentially force her to pay all court costs and attorney fees. In this case, she is the one who would be violating the order; not you.
So, I would document all of this and do some research along the way. If the visitation even becomes an issue, politely send her a "certified" letter reminding her of the order. If she still refuses visitation, you have proof that she did not comply and you tried to resolve it. That will be an open & shut case for sure.