your petrol level dropping problem and petrol smell in cabin is due to a leak somewhere in the petrol system, a hole in the fill or breather pipe will also cause that.
The engine cannot emit black on CNG as methane molecules are very small compared to petrol, and it being a gas - if you fed it more the engine would balk and shut down (try it by opening up the power screw - the engine rpm will drop to the point of engine shut down).
Your oil consumption maybe due to bad valve seals, to verify, drive the car around for 200 kms and check the spark plugs, if they are the correct heat range and the valve seals are bad, the plugs will have crusted deposits on them. If the rings are bad causing low compression the plugs will be oily black like a dead 2 stroke engine has.
The smoke emmitting at high rpms is residual soot in the exhaust mufflers, If you know that you never heated the engine to H on the gauge and did oil changes at about 5000 kms intervals no matter with what oil - and the PCV system is clean (no sludge in the engine) - your rings would probably be OK, these honda engines are great in this regard. Get the valve seals replaced - if your engine is consuming too much engine oil. A good mechanic can remove a Honda SOHC head without removing much parts.