Knockdown and critical success on whip

[Particle_Man]

If I use the 2 action knockdown maneuver but on the strike I get a critical success and being a 5th level human fighter with unconventional weaponry and mastery in, say, gnome flickmace, make the opponent prone on that strike, then there seems to be nothing for my second action to do since I can’t try to trip someone that is prone already. Does that second action still get used even if it does nothing? Does it still count towards a multiple action penalty?

[Draco18s]

Knockdown is a 2-action Activity. You committed both of those actions when you started the Activity. The critical success on the strike portion making the second portion irrelevant doesn't mean you get the action back. It's already been spent, same as if you had cast a spell and got disrupted.