"why IR will term Passenger Service as Mail"

Because there was no IR at that time. There was no IR till 1947, no railway zones till 1951. Before that different companies operated the railways and each of them ran their railways in the way they saw fit.

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keep talking about mail/exp and passenger - have you done any research into what the type of trains were in the 1800s in each of the railway company? Just because EIR marked some train as mail, doesn't mean that every company had the same method.

Attached are records of the Madras postal department from 1864 - Note that they are carrying mail by multiple trains every day - not just one train. The trains that you keep calling 'pass' also carried mail. In fact, not just one train, but three trains carried mail from Madras - perhaps all of them should be called mails - The Tripatoor 'pass' from 1861 that you were talking about - that also carried mail.

The concept of fast, priority mail trains only came up in the 1880s, when they started speeding up existing trains by removing halts and making the 'mail' a prestigious term. Before that, any train that carried mail was called a mail train on most railways.

Also note that they did not have overnight trains in 1864 in Madras. The mail train stopped at Coimbatore (now Podanur) in the evening and continued the journey next morning. Before you jump to the conclusion that these are separate trains - attached is a record of the medical department of the Madras railway on Coimbatore (now Podanur station) from 1865 - they mention that the trains from the east and the west remain in the night at Coimbatore and continue the journey the next day.

And as for why the timings changed - 160 years have happened since these trains have started running. Is it realistic to think that trains will run with the same schedule and timings and numbers for 160 years? Lot of changes happen. The Calcutta - Delhi train in 1868 was taking close to three days for the journey, compared to 25 hours now - so should I say that they are different trains.