I think spears should be able to trust from 4 directions, up, left, down,right, shield or no spear. For those spears and polearms that can slash, you would just press X to change weapon mode. Same should apply to all polearms, even some axes have a sharp point that could be used to thrust, if not very well. But slashing with spears should definetly only happen when holding it 2 handed.
Though it should be much faster to press X in some situations. Its fine to have delay if you have to change grip, like with throwing spears which have cool detail of character actually flipping the spear right way around.
Not sure if 15 swing damage for spears would be realistic number