Contempt/Mod of Support Order

My question involves a child custody case from the State of: Washington

I have a support order from 2010 that needs to be adjusted. due to a loss in my family I had to move back home and take a lower paying job.

Because of this pay decrease I wasn't able to pay my full support amount and began accruing back due support.

Also, our support order states that I get to claim 1 of our 2 children every year on our taxes, however she isn't allowing me to claim her and won't sign the required IRS tax form. It's been 5 years of tax returns and i haven't been allowed to claim her on any of them.

So I'm wondering if I can file for contempt of court and request that my back child support be removed in repayment for the loss I've taken in tax returns. As well as requiring her to sign the tax form.

Or

Do I petition to modify our support order and include those requests within the modification paperwork.

I'm trying not to go to court twice and would rather just have everything taken care of at once but don't know if it's possible.

Perhaps she will agree to removing back child support rather than being required to amend her past tax returns but I doubt it.

I've shown her the laws regarding the taxes and informed her that she's violating the court order but she seems to think that being the custodial parent means she's untouchable.

Any help is appreciated.

Contempt/Mod of Support Order