No. Redmi 6A does not have Dolby Digital sound support.
Yes, Redmi 6A does have the face unlock.
Redmi 6A has 2.0 GHz MediaTek Helio A22 quad-core processor.
Yes, Redmi 6A does have 3 slots - 2 for sim cards and 1 for the MicroSD card.
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.