We can narrow our collision searches using pre and post image ideas. What you have to realize is that a hash function is only a N-ary tree where the data being hashed is used to walk it. With hash functions we are provided the path to follow and use that to determine the final node, sometimes we have previous knowledge of the node we are supposed to be at. It is merely a state machine with a larger than normal set of states. We have only 1 measure of strength, whether or not we can compute a path to a given endpoint, and how long it will take.
Any assumptions otherwise are misleading.
I'm not quite sure what you meant in this XML fragment. It was a suggestion of one way to imbue both unknownpre and unknownpost images into a signature. There's a lot of cryptanalysis behind using a method like that, regardless the attacker still only has to guess the entropy content which may vary widely.