I must take most of this back... I did more PvP and found the issue.
In Dark Souls, when I couldn't/didn't want to toggle out of a combo, I'd block instead of rolling away (I just got used to that). Here, since blocking isn't instant, it doesn't let you escape a stunlock. If you mash roll instead, you escape it, and I've been enjoying PvP much more since I realized that. The lack of backstab fishing is refreshing, and the guard break critical is incredibly fun. All weapons still seem to break too much poise, though. I know some degree of poise works because my arrows didn't stagger several of the people I invaded, but melee weapons seem to break a ridiculous amount of poise.
it seems PvP poise is kinda broken and you were right that poise for PvP is ignored or somehow totally different. i've come across a couple threads on gamefags (lol

btw, why did they remove the aiming for spells? i've seen a couple of people complain they did that, and ENB's 2nd vid confirms they did that. are strong casts and "hold casting" also removed?