Yes, Nokia 1 have the removable battery.
Nokia 1 is not completely metal body. It has metal chassis frame and the housing is made of polycarbonate material.
No. You cannot use 2 4G SIM cards at the same time in Nokia 1. One SIM card will work on the 4G network and another will be on the 2G/3G network.
Nokia 1 has IP52 certification. It is not exactly waterproof but it does have drip protection.
The Nokia 1 is an inexpensive Android smartphone, and as you can tell from the Nokia 1 review, you very much get what you pay for.