Is agentive activity included in every representation (intension)? If so the extension of every representation includes some information about the interactivity between the representing agent and the thing being represented. For example: iconic, visual concepts (what a dog looks like) presupposes the activity of visual perception; namely looking at a dog, paying attention to certain salient features, committing this to long term memory and so on. Or to put it another way, you can’t have information about some x, without some amount of interactivity (A1,A2,…,An) relating agent y to x. vis., [A1->n(x,y)]. Does this not imply that there is no such thing as objective knowledge?
To break this down a bit, consider that "having objective knowledge" can be construed in two ways. (1) You can "have objective knowledge" in a limited sense in which you are simply correct in asserting some statement, without knowledge of whether such statement is in fact true or false. I think this must be possible in a sort of "infinite monkeys on typewriters" type of way: given an infinite set of statements, at least one must be objectively true, therefore objective knowledge is possible. But what I think you are really asking is (2) can we know that we have objective knowledge? That is more problematic. This was Descartes' quest in Discourse on the Method: is there some basic proposition that we can definitively say is true or false?
This question cannot really be answered because answering it involves oneself in the question itself. If we say, "no it's not possible," then we have set that proposition up as objective truth and thereby contradicted ourselves.
I think it would be useful to consider what your standard of proof is. Objective knowledge in mathematics is more strictly defined than it is in the empirical sciences, which is more strictly defined than law, which is more strictly defined than history.
To address your point about agentive activity: it does not really matter so long as the resultant proposition remains true. That is, as long as you can account for the effects of agentive activity, then you can still appreciate it objectively. Or perhaps it has no effect at all -- you seem to have assumed that it always does. It is true that since we are finite, instantiated beings, we always posses a point of view or vantage point; yet it does not follow that there aren't certain vantage points from which an objective assessment can be had.