wow u have thought of everything, so get 11.5kg cylinder n then keep getting that replaced instead of the 15kg one
though i did asked him if 15kg is the minimum pre filled, n he said yea its minimum
u got some 12kg pumped at the rate of 94 or u just bought full 15kg
i guess once lpg cylinder is set in trunk, u can easily remove the pipe of cng yourself, remove cylinder get it filled, place it back n join back with cng pipe?
on cng one km costs around 5.6rs where as petrol it costs some 12-13rs, i guess if u want to remove cng pipe from cylinder n shift to lpg cylinder then cylinder should be empty esp if u wanna fill lpg in cng cylinder, that means travel to either of places on petrol
21 x 12 = 252rs n then back on lpg, again some 5 x 21 = 105
for test purpose may as well do it, lets just see how much average you get in local drive, so u have filled 15kg for local drive?
my dad did approved this experiment of trying lpg domestic cylinder like u did but to be tested not before mid june when i'll get free n until u'll have done some local drives too 