1) yes Hindus are Najis as per the ruling of Aagha e Seestani if it is touched by kafir with wet hand then it is pak but In above situation it is hard to say that it would not be touched so consider as Najis. Because Chef, waiter all are Hindu that means there is not any possibility of the taharat .
2) If restaurant own by Muslim in front people are Muslim then do not investigate it is palak.
3) Mentioned chains are helping Israel so refrain from then it will be help to oppressor. If it is known that workers are non Muslims then off course then not allowed to eat in the light of the answer of the first question