Why we go for ENA instead of ENI ?

Because ENA is Supported via EFA extension

Is it possible to attach multiple Network interfaces to single EC2 instance ?

Yes it is possible

can we copy the data from local computer to Instance which logged in with private IP ?

yes

Can we connect Multiple VPC's through VPC peering ?

Yes, b using Transit Gateway

What will happen ,if we remove the peering but didn't remove its corresponding route ?

route ramins in the route table