Interesting idea.The riser tube is normally cut-off flush with the top of the tank. Since your riser tube extends above the tank, it could become deformed from downward force when the control valve is threaded securely onto the tank.
How does the depth to the shoulder in that adapter socket compare to the flat surface at the base of the big threads?Hmm...ok, so you think if I am able to cut it flush it might be ok? I did notice the top of the tube looked slightly deformed....but not a lot (you can see it in the picture that I posted where I measured it). I'm not sure if that looks like enough to be causing leakage.
Some digital calipers have a depth gauge that extends out as you open the jaws. Otherwise, you can insert a dowel and measure a mark on that.