Removing the hydrazine byproduct from the mitsunobu reaction
Acidic wash is not going to help. As you said you can try a DIAD alternative for example DCAD can give good results (sry no advertising intended here). There is also the option of taking your contaminated product to the next step(s) and hope you can get rid of the impurity there. I would take option A if I have to make a bunch of similar analogues by a Mitsunobu and B otherwise for not wasting too much time