Is this correct?

Yes. If child is 18 he/she is beyond court's jurisdiction.

If a petition to modify child support for an eighteen year old is sent to arbitration, can still file a motion toward case?

If your case is filed in Snohomish county then child support modifications are subject to Mandatory Arbitration, if the only relief is money, thus...

The initial question is did you and your ice live in Washington during marriage? If not Washington does not have jurisdiction if your wife. If all...

Where is the justice?

Child support as income driven. It sounds as if your wife is it voluntarily under or unemployed. Unfortunately, she has insert real work history....