You definitely can use compressed air to reform the tank, but you do not need that much pressure. I had exactly the same problem (after replacing the fuel cap with the non vented version!) and I used a standard foot pump to pressurise the tank and reform it. If you were sticking 10bar in from a compressor it may be an issue, but carefully pressurising it to expand it out is not a problem.
What colour is the fuel filler cap, the vented and non vented version are different colours.
There should be a plate under the tank called a cradle, to hold it in place. It is not strictly a bash plate as is only thin and has a hole in it around the drain plug so it can still be accessed. IF is is damaged from impact a proper tank guard will prevent it in future, but I am going to side with everyone els eon the thread and say it looks far more like it is creating a vacuum from a blocked (or non existent if the wrong cap fitted) breather.