dollhouse is right, 4-wheel aligment is the right way, no matter if it is turning to the left or shakes after high speed. (Shaking while parking is a different case, that is a need for spark plug replacement, or maybe wires too.)

you said you have done the aligment, then you must have the aligment paper they give to you, what does that say? all the numbers arent suppose to be too off. Usually after the aligment, car should go stright w/o holding the wheel. Unless the car had been in a serious accident where mech had to "bend" the frame back to "normal", which isnt normal.

also, check left and right tires worn out tracks. if they arent even but still kind of new, maybe just rotate them too, so they can have a even worn out, then replace the tires when time comes, then do the 4wheel aligment

About the gas leak, hard to say, i am not so sure. Try to find where the smell come from la? can you? yaosen is prob right. But get a free estmate from few shops are something i would do if you cannt find where the smell come from.