Yes, Redmi 6A does have the night mode. It also has eye protection mode.
No. Redmi 6A has 2 dedicated sim card slots and a dedicated MicroSD card slot. So, you can use 2 sim cards and a memory card at the same time.
No. Redmi 6A does not have the NFC support.
The Redmi 6A is a fairly good update to the Redmi 5A, which itself was barely any different compared to the Redmi 4A. You now get a tall screen, better processor, and improved polish overall.