Yes, Redmi 6 does have a fingerprint scanner. It also supports the face unlock.
Yes, Redmi 6 is a VoLTE phone.
Yes, Redmi 6 does have the 18:9 aspect ratio full-screen display.
Yes, Redmi 6 does have the dual apps support. You can have 2 WhatsApp and 2 Facebook accounts at the same time.
If your budget absolutely can’t exceed the Rs. 8,000 mark, go with the Redmi 6 without any concerns whatsoever.