From my experience using PayPal for selling products on eBay (and for the last two, experiences of a friend)...
Can paypal get money from my Bank Account without my authorization. This is assuming they have transferred the funds to me.
They can't pull money from your bank account without your authorization. They will, however, take the money from your PayPal account if it's still there, or leave you with a negative balance if you've already withdrawn. They will do this as soon as there is a claim against you and will only release the funds if the investigation ends in your favor.
Any money received would first be used to satisfy the negative balance.
What actions can paypal takes against me if charge back amount is very high and I don't agree / pay them.
They will send it to a collections agency.
Is there any case it is going to effect my bank account, i.e. is there any chance paypal can block my bank account in India.
They will block you from using PayPal. If you try to sign up again with a different bank account or credit card and they recognize you as the account holder, they will block that account as well.