Using sr036 only for the regulated output
I have never used this device, but most likely, the 18V unregulated output is the sole input for the regulated supply, and not the high voltage input. This looks like an essentially linear device, and it is not practical to regulate what could be 300V (pk.) down to 3.3 or 5V using just a linear regulator.

This can be seen in the diagram at the top right of page 4:



So, yes, you do need the switching transistor, otherwise there will be no unregulated output, and hence no regulated output either.