BMDs are for IRBM only, not ICBM. Indian BMD like AAD is for MRBM only (<3000km). As of now, India can intercept missiles with range of 2000km and intends to develop it till it ca intercept 3000km missiles. S400 can intercept missiles with range of 4000km.
No one cares for your submarines. India can make its own submarines in any number to counter Pakistani ones. Also, Pakistani submarines have to surface every now and then to refuel which will make it easier to detect and track.
Every SAM, BMD system uses multiple sensors. Also, the kill probability is never 100%. Even if it is 60%, firing of 2 missiles will make it 85%. That is good enough to reduce the number of hits by about 85%
Suicide drones are also called cruise missiles in its simplest form. There is no point in having drone swarm.