1) Actually, Sartre, like Heidegger, does away with the object-subject dichotomy: The for-itself cannot be in-itself, but this relation is not the same as that between the subject and the object, which is so drenched in faulty metaphysics that it precludes a true view of the matter at hand. The for-itself and the in-itself should be considered theoretical abstractions of a whole (being), a mere methodological device for the sake of simplifying analysis, and the "relationship" between them is a much more intimate one. As Sartre says, the for-itself is being nihilating itself.

To answer the original question, I think it might be better to start with the "relationship" between consciousness and itself: Sartre talks about the pre-reflective consciousness as the way in which consciousness concerns itself, and signifies the relation by putting the "of" in "consciousness of itself" in parentheses: Consciousness (of) itself.¹ This is to signify that while consciousness cannot exist other than as consciousness of something, it also implicitly maintains its own structure as being conscious (of) being conscious of something. This is why we so readily answer "I am counting" to the question of "what are you doing?" This relation of consciousness to itself is immediate.

In opposition to this, consciousness of something is consciousness of not being what it is consciousness of, and as such it is a negation of what it is consciousness of. In other words, the relationship is a negative one. This is how "things" are differentiated, how something can be fragile, etc. This relation is mediated.

2) Both the conscious and the nonconscious are part of being. As I said already, the "relation" between for-itself and in-itself is not really a relation as such, at least not if a relation implies distance; they are aspects of a whole, of being. Consciousness differs by being that which brings nothingness into being, or in other words, is being bringing nothingness into itself.

3) I'm not sure where you get that bit from, but if Sartre says that the nonconscious cannot understand nothingness, it is because something nonconscious simply cannot understand at all. If you mean how the in-itself cannot admit nothingness, the point is that if you abstract being into something that is principally not witnessed by any consciousness, then you cannot say very much about it, but you may approximate it by saying that it coincides perfectly with itself, that it doesn't allow for any nothingness or distance, etc.; the in-itself simply is, neither more nor less.

4) Sartre's concept of "self-deception" (I take it you mean Bad Faith) is one where we take a reflective attitude and direct our consciousness on itself in the way in which consciousness is consciousness of other things. In other words, if consciousness looks at itself and takes itself as thing, we are in bad faith. It is very "easy" to make this mistake because essences are constituted through successive appearances of anything such that whenever we take ourselves as object, we run the risk of confusing what we might be said to have been with what we "are."

1: I can't remember exactly, but I think in some places, the English translation turns this into "self-consciousness" because the translator believes this English construction is a good way of conceptualising Sartre's thought. However, I think the only result is that, as "self-consciousness" is a more established construction than "consciousness (of) itself," the phrase is simply passed over as another word in the continuous stream of a flowing text: Much like how in "paris in the the spring," we skip the second "the" (at least when the sentence is properly formatted -- it might not work here), we end up reading "self-consciousness" as [self-consciousness], and not [consciousness (of) itself].