Why end with plural hain and not hai?
To show respect. Its an honorific. This type of ending is found in a lot of place in Hindi when elders are involved.
"Mere Papa aa rahe hain" literally translates to mean "My fathers are arriving" but actually means "My father is arriving."
Because for for people of respect, we use the plural hain instead of hai
Why not 'dada andhar se hain?'
अंदर andar اندر is a Persian loanword