Hmm, to many doubts like a school boy
I like it !! see my answer below each question which are best of my knowledge.
1) within 12 days of purchasing,i have done almost 1000 kms. but only on cng. should i drive it on petrol also sometimes?
Lot of quick running buddy, it is not mandatory, but use it often to check the performance difference.
2) any suggestions on seat cleaning good quality brand liquid,dash board cleaning liquid,and other interior cleaning options?
Check if you have Reliance Auto zone or something like that and you will get all those things
3) the car is due for 1st service.is it really free 1st service or will i have to pay something?and if yes then how much?
Only consumables and spares(if any) would be charged, so it will be basically for engine oil if it is specified in the schedule and some top ups may be, it the charges will be minimal
4) the accessories that i have installed, will it affect my car warranty?( music system,seat covers,led lights, floor carpet lamination)
NO, they do not void warranty as per the list you provided
5) as far as i know ,sun films are not permitted. but are they totally banned or we can put 50% transparency sun films at least?
It depends on your city cops checking situation, if you really want them, then prepared for removing also when a cop objects, try to go with simple RTA approved V-cool,garware,LLumar films
6) many times my engine gets switched off while moving ahead in 1st gear.(i know i may be taking my foot off the clutch pedal too early but is that the only reason?)
Nothing to worry, you need to get used to the vehicle and clutch usage, if you have any issue, then tell the service guy then he will adjust the clutch
7) should i start the engine by pressing the clutch pedal or i can directly do it without pressing it?
Not mandatory