Am I entitled to claim full end-of-service benefits—including salary and allowances—according to the labor law, given that the signed contract stipulated their calculation based on the basic salary only? Would this be an injustice to the employer?

It appears that since you willingly forfeited your right, which labor law guaranteed you, you are not permitted to sue the company and demand what you agreed to relinquish. This is because Muslims are bound by their conditions, and you are held to what you contracted.