The situation is almost so ideal for Amazon that I wouldn't be surprised if someone high up in the company planned for this to be the location to try to unionize in order to poison the whole concept. They won't even need to play games with shutting down the warehouse as union failure at this location is highly probable.
Is everyone required to join? No. Following Janus, they might not even be required to pay dues. However, the union contract will apply to everyone in the bargaining unit (generally all non-management) regardless of membership.
Is Amazon required to negotiate? Yes, they're legally required to make good faith negotiations. Whether or not they actually do this is an open question, as companies regularly stall here.
Can Amazon simply close the plant? Not legally, no. They would probably end up owing the workers quite a bit of money if they did. Whether or not they take that course of action may be a business decision.
I think it will be very interesting to see what the unions wants:
* cash raises (and what sort of structure)
* non cash benefits (health etc, more breaks, shorter lines to clock in and out)
* guaranteed hrs
* job security
* political faff
If they vote yes, is everyone at this location required to join? Are amazon required to negotiate with the union? Can amazon simply close the plant?
Presumably if unionisation works for these workers (better pay etc) then other centres will follow suit...