Reading Group: Derrida's Voice and Phenomenon Re: the last paragraph, it's aim seems to be to indicate the ramifications of Derrida's investigation, an attempt to explain how this seemingly trivial point about indication and expression has implications beyond what is immediately obvious. In particular, it bears upon "all the reductions to come, whether they are eidetic or transcendental." The whole first part of the paragraph (up to the words "And yet Husserl... (middle of p.26)) is just a series of ways of saying the same thing. Every 'reduction to come' will be affected by this problematic (I don't want to say problem), such that "indicative signification will cover, in language, all of what falls under the blows of the “reductions”: factuality, mundane existence, essential nonnecessity, non-evidence, etc." The bit about the 'hiatus' between indication and signification again, just says the same thing.
At this point (after 'And yet Husserl...), Derrida indicates that there are 'two paths', as it were, that one can follow at this point, paths opened up by Husserl himself. Husserl opts to follow one path, but Derrida signals his intention to follow the other. One is to follow Husserl in simply bracketing indication as something that must be taken into account, only be to put aside, as it were, in following the travails of expression. The other is to ask what would happen if we take indication to be intrinsic to expression itself, if, by necessity, it 'contaminates' the purity that ought to characterize expression. Derrida clearly opts for this latter understanding: "Although there is no possible discourse without an expressive kernel, we could almost say that the totality of discourse is gripped by an indicative web."
You can start here to see Derrida's complex relation to the phenomenological project more generally; Derrida never claims to be engaged in a 'critique of phenomenology'; rather, he always generally takes himself to hewing closer to the foundations of phenomenology than Husserl himself. Elsewhere (I can't remember where), he will speak of the necessity of the phenomenological reduction as a starting point for philosophy in general. Len Lawlor sums up Derrida's strategy thus: "Derrida argues that every time Husserl tries to define the transcendental without the empirical he fails, necessarily, to be rigorous. The transcendental is contaminated by the empirical and vice versa." This is the program that will be pursued in the following chapters.