Can I reopen a divorce case for more child support?

Yes, you can request the Court reexamine child support (whether you need to show a material change of circumstances depends on whether the court took actual testimony or support was set by the parties). In general a court will reexamine support every 3 years or when the child reaches 7 or 13 years of age. The Court will also review support based on significant changes in income or jobs or when a party seeks to move away from their present residence and it would be harder upon the other parent for visits. You are probably not going to be able to claim additional or new alimony unless the court reserved jurisdiction on that issue. On the tax issue, a motion to enforce the parenting plan would be most effective if you can't resolve your issue through informal mediation.