No. Redmi 6 does have a dedicated MicroSD card slot and 2 other slots for SIM cards.
Yes, Redmi 6 does have the ambient light sensor.
You can record up to 1080p resolution videos with Redmi 6.
Yes, Redmi 6 can support 2 4G SIM cards at the same time. It is a dual 4G phone. It also has VoLTE support.
If your budget absolutely can’t exceed the Rs. 8,000 mark, go with the Redmi 6 without any concerns whatsoever.