I think the original post is a very telling attempt to engage with deconstruction in that it seeks to establish what a deconstruction is
when in fact, for Derrida at least, deconstruction is what happens
to a "text" - that is to say, it cannot be forced into taking place. This is why Derrida was very uncomfortable about the way in which the term deconstruction was taken up by others and turned into a set of guidelines for analysis or critique (we might call this deconstructionism).
One way to understand this could well be through Simon Critchley's description (in The Ethics of Deconstruction
) of the process of what he calls "
reading", which is a double reading that seeks both to remain faithful to the source text while at the same time offering a radical engagement with its ideas (the word
refers to the so-called "closure of metaphysics" which Derrida sought to expose as the moment when seemingly transcendental philosophies revealed themselves for the duplicities they inevitably were). Critchley's approach also draws on Derrida's engagement with Emmanuel Levinas and his ethical philosophy of the Self-Other relationship (intersubjectivity), which in textual terms can be seen as the distinction between "the Saying and the Said" (hence the necessity of a double reading).
would also be valuable here, but I am thinking about that in a different context so I will leave that aside for now on the proviso that I shall return to it later. However, just to say that the French term
exists no more in French than might the transliteration differance in English (hence why Derrida said it is not a word or a concept as such), other than in reference to the double meaning (and again, double reading) of the French verb
as both difference in space (difference) and time (deferral). The point being that it does not function in language in the same way as the distinction between whether and weather suggested by the original poster - i.e. it is not simply a question of determination.
Regarding presence, and specifically the relationship between speech and writing, it is commonly accepted that Derrida sought to undermine what he called "the metaphysics of presence" - the privileging of the present as the source of intended meaning. In a certain sense this is quite correct, in that Derrida emphasised that language is only capable of functioning on the basis of the traces of difference-deferral which he called differance. The essential point is that the "metaphysical" approach is inescapable because we always have to use language and therefore are "always already" destined to mis-speak. However, while this can be taken to suggest Derrida's privileging of writing over speech, nothing could be further from the truth (in fact, many of Derrida's "writings" began as oral disputations); rather, in Derrida's philosophy speech and writing are conjoined twins, thoroughly inseparable from one another without an act of extreme violence (hence his abiding interest in literature, which is a sort of middle ground between speech and writing).
So, to conclude briefly, Derrida's philosophy does undermine concepts such as truth and presence as they are traditionally understood within the philosophical canon, but not by dissolving them into a facile relativism. Indeed, Derrida counsels a philosophy of affirmation rather than negation, an openness to possibilities and not a closing down of boundaries.
Hope that helps!