Additionally any engineer should be able to verify if the wing structure can withstand the weight of the fuel.

It doesn't have to. The wings are either flying (supported by the air) or supported by the wheels. In fact it's the wings that are supporting the rest of the aircraft.

On the other hand some flight tickets are way too cheap for the proposed fuel consumption.

Flights that are low cost are simply because of gov subsidies. Cities want tourists, that's why there are low costs flights.