EDIT: BTW, would it be possible to have on/off switches for the separate features, i.e. like set PPP.LessKnockback 1 or 0?
Stagger has nothing to do with knockback, neither does PPP affect stagger behaviour in any way. Knockback is what you can see in http://www.youtube.com/watch?v=KmOaax84O6M video.
Zombies are affected more than most other creatures by it (in vanilla) since their agility is extremely low, which is what the knockback reduction setting you had problems with controls. That setting is a bit stupid since not all big creatures have high agility (since you'd expect them to be knocked back less) and not all small creatures have low agility (since you'd expect them to be knocked back more). That's why we decided to make the knockback effect depend only on the amount of damage done and generally make it more subtle than before. Low damage will cause (almost) no knockback, high damage will cause a noticeable, but not ridiculous looking knockback effect now independant from the agility value of the target. In vanilla you always get a knockback effect and for creatures with low agility it looks pretty ridiculous like can be seen in the video (especially on uneven terrain - Michael Jackson is alive!).
I guess it would be possible to have a switch, but some features already have a switch like that and the other ones only have a few settings that you need to set back to vanilla or to 0 or -1.