No. Redmi 6A does not have Dolby Digital sound support.
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.
Yes, Redmi 6A does have the face unlock.
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.