I have problems comparing the flip mode of some 1h and polearms with halfswording. The flip mode only offers a comparably weak alternative to the main mode, while using the same animations. It doesn't change much.
On the other hand halfswording in its planned form changes the animations and allows it to be used in cramped space. It is a completely different fighting style. Furthermore 2h swords are already very powerful with their main mode alone. So unless you want a big nerf for their main mode, halfswording stays polearm.
I would compare halfswording more to the 1h/2h hybrid weapons. I have a char with mixed 1h/2h wpf for that, pretty funny.