Your money your car your choice.
I spoke of mechanical repair learning/education and experience. As I said - when you are in drive anytime you let go of the accelerator the sprag takes away the engine braking which is what worries you. The transmission is already hypermiling for you. The reason it does not come down back to idle speed is because its connected to the driveline and the torque converter has stalled its turbine.
If you think you can save money by hypermiling then go for it. I personally have seen a lot of transmissions ruined because people would switch off the engine at coast down a hill. Good for transmission repair business - it puts bread on the table for their families.
In all honesty if you are that much strapped for money that you are pinching off a few scant paisa then its better to make more sensible decisions like do you actually need a car that has a certain buy and running cost? why not chisel down on the whole package, e.g.
new car = 1,300,000
run = Rs 30,000/month
average ownership - 5 years = 1,800,000 = fuel.
depreciation = Rs 300,000/-
why not work the whole figure out and be more sensible - rather than going down the same path of agha waqar/cng mania/etc. A vehicle is a work tool which has a certain cost to it.