how exactly did ur mechanic check the coils? i highly doubt ur mechanic's statement. first off, if they are leaking, they would leak on petrol as well as cng. secondly, it would not cause missing, when the coils leak current, one or several of ur plugs dont get current and hence no ignition. when theres no ignition, those particular cylinders dont make any power for u and because the air fuel mixture is not burnt (its called misfire) , when the cylinder comes back up it pushes the mixture out of the INTAKE valves , if this happens, you ll hear a loud bang from ur air intake and the force is enough to break ur intake manifold or atleast crack it if its made of plastic. after a second or so, ur car will behave normally and after some time this would happen again. but this is not contineous.
Secondly, its highly unlikely that the front ones are leaking any current, they are toooo thick to leak any current. my mechanic says he has replaced hundreds of rear coils of city idsi but not even one front coils of any car. plus, at just 106k kms, its highly unlikely that ur coils will be dead unless u got water in the or sumthing. (if u bought ur car 2nd hand, may be the meter is tempered)
i'd strongly recommend u to go to some other mechanic and just tell him to increase the idle speed of ur engine from ur cng kit. that ll fix the issue hopefully.