There is something wrong or missing in your post or your union has turned gutless and rolled on you or something --just my old HR views--it the contract mandates arbitration withing 180 days --and company has refused to arbitrate then it seem to me the next logical step is a court case to mandate your reinstatement with full back pay, benefits etc as the company has failed to follow the mandated temination steps
Fired you for what?
Generally, your remedies are limited to those in the contract--and if your union has put you aside and has other fish to fry--you are sort of out in the cold UNLESS the union picks up the ball and forces it back upon employer. Union has considerable latitude as to which fish it wants to fry--and if you and your cause are 'expendable"--well, you are history.
This may be 99% about internal union politics...So how do you get union to put you back on front burner?