Spoilers!
There are still some things that weren't answered:
1. Does Lalit know that they are all going to die and he is just trying to use possession as an excuse to control the family? It was said that the responsibility of taking care of the whole family fell on his shoulders. And he used possession to easily control them and when he couldn't take the stress anymore or thought he couldn't control them anymore, he just did the mass suicide. Made up the entry in the diaries about instructions on "untying" each other". Did this controlling behavior really start after he had the concussion? He may also have made up about losing his voice and used that to control them later having the "miracle".
2. Is he really sick and thinks that he is possessed and believed that they are going to survive the ordeal thus the diary with the instructions on untying each other after.
Yes all of them educated but they all come from a tight-knit family and in a family like this it doesn't matter if you are educated or not. It also didn't help that none of them had a dominant personality except their father who passed away. The family dynamis is probably one of the reason why Dinesh didn't want to stay. He didn't want the responsibility..
Very sad for the rest of the family who didn't know any better.