Hi, I've been going through this thread and i must definitely add to what the other members have said and also what you have realized by now, change your riding style that meddling with the works of your bike.
If your an aggressive rider in the city then slow down a little, try and avoid unnecessary revving, unwanted speed bursts on ending signals or over taking. keep your engine off at signals, dont exceed 3.5K on the rpm, be smooth with your acceleration as well as deceleration and last but not the least start early to work when traffic is at minimum this way lesser congestion in traffic, lesser gear change, more mileage !
these things may sound simple and small but believe me they make a huge difference. i ride a ZMR and get 45-48 kmpl in Bangalore city.

as for your concerns with filling nitrogen, sorry but it doesn't make any difference with regards to mileage, though you'll have the benefit of better tube life, smoother rides and if your using tubeless then the risk of rust is much much lower as its an inert gas.

i didn't understand the whole 100ml mileage test. if you meant he gave u a 1 litre bottle (1000ml) and you tested it by putting the bike on main stand and running then thats the most ineffective method as tyres have no friction, no rider weight and no real road conditions. if you filled in just 1000ml and got 51 on it then i doubt your mechanic had drained entire tank before giving it to you for the test.

as far as cheating petrol bunks are concerned, can't help it, you got to be very careful about where you fill, best advice to make sure they are filling while meter is running is, keep you hand on side of the tank and eyes on meter, you will feel a vibration on your hands, as long as vibration is there you know he is filling when the vibration stops and if you notice that the meter is running then something is fishy. i don't know about AP but here in Karnataka i don't trust Indian oil petrol bunks at all.

Very impressive figures there but not a surprise after you changed the spark plug, extra psi, etc...

And that extra psi could have landed you in trouble. If you filled up the 3-4 extra psi before starting out on your journey, your tyres would have heated up during the trip resulting in the psi coming up further (1-2 psi i'd guess).

If you take a low down turn with that sorta pressure or if you encounter a wet road or oil spill... Down you go since there's hardly any tyre gripping the road!!

my mechanic has filled 100ml of petrol from my tank into a small bottle and attached that bottle to the carb. And we went on a small ride until the 100ml in that bottle was over. I was able to do 5.1km with 100ml which is 51km for a litre of petrol. In this way he told me that bike was working perfectly well.

Yes this is a standard testing method and gives a true MPG...if all others things remain normal......
In real world situation of stop and go traffic ,adulterated fuel,short fill by petrol bunks,etc., you can hardly hope to achieve it unless you are a highway rider for most of your ride,with easy traffic and no frequent stops....and you ride at a sedate speed

Was the carb dry of fuel? if not you got a figure including float bowl capacity. Even then such conditioned test results are not real. Test for atleast a liter in your usual ride conditions. i had a pee150 which on a bajaj ASS test give 32kpl while i used to get ~42. Well the looks on service chap was all PRICELESS