Question about clearance/pad diameter and PCB design
The datasheet specifies the clearance and creepage distances the transformer provides.  You do not have to maintain or exceed these exact distances in your PCB design.

If you want to maintain the creepage distance, you can insert a PCB cutout between the +/-HT pads.  This will increase the shortest path between both pads on the PCB, leaving the transformer the shortest path, at 19.5mm.