All this really depends on just how much skill in each it takes to gain access to the moves
If its similiar to backstab at 200ish then I should think that atleast all high level melee'ers would be able to use the 1 sure parry but not gain any bonus ones without spending IP. Personally I hope its a bit higher so that trickle down isn't enough even at 220, so you have to aim for it.
As for riposte, im guessing it will either be a % chance to be enabled per attack or specials trigger it, but I doubt evading the attack would stop it being opened.