Since you are convinced that the marines are innocent, present an Italian government lawyer to take the case on your marines behalf, let the case run without too much media attention and let the verdict come.
If they are innocent, they will definitely be pardoned and sent back. The law here is based on democratic principles and not run by some tinpot dictator's whims.
So your marines are safe from any bias.
Escalating this will only result in fanning needless attention to other elements which is counterproductive for both Italy and India.
There is a big difference between near Indian shores and waters and in Indian waters. Please understand that we don't allow pirates to come in and ensure that it doesn't happen. Not anyway near a key city like Cochin.
You say innocent, we say guilty.
The simple solution is; let the law decide. We can welcome an Italian lawyer to fight the case for the marines.