If your citation is
WAC 173-62-030(3), it prohibits explicitly the sound (squealing or screeching), not the tires from spinning. So your sworn testimony that there was in fact no squealing and that your car is equipped with traction control that is designed to prevent wheel spinning, along with the claim of wet pavement, which will hopefully be corroborated by the officer's testimony, should carry enough weight to get you off that charge. Given that the traction control need not "prevent all spin", just enough so that there is no squealing noise, you have a good chance, IMO. Provide some documentation reg. your car having said traction control system... I don't think you need "expert witness statements." As for the seat belt, I see little hope in avoiding that charge. Good luck.