2 goes.

'There is a logically possible world physically identical to ours in which the positive facts about phenomenal consciousness in our world do not hold.'

This is what Dave C means. Negative facts about phenomenal consciousness are facts like these--the desk has no phenomenal consciousness,
the shoe has no phenomenal consciousness, the rock has none, and so on. THESE facts about phenomenal consciousness DO hold
in the world without phenomenal consciousness. 'Positive' facts about phenomenal consciousness are facts that this or that
thing has phenomenal consciousness, e.g. that Stone has states with P-consciousness right now.

You write:

'The Duck Story:

As things actually are I am watching a duck swimming on a pond. Not only that but I am in a phenomenal state associated with my duck watching.  That is to say, [roughly] there is something it is like for me to be watching the duck in this way [?}'

Right. There is something it's like for you to be aware of the duck. But we can at least TALK as though being aware of the duck is something a physical system does,

a functional state, whether it has experiences or not. So the zombie is also aware of the duck, though there is nothing it's like for him to be watching the duck. 

You and the zombie are in the same functional state, you alone have an experience, and we can say that you are both aware of the duck, though only

you have an experience.

This is a sensible way of talking, Chalmers has no problem with it. We can refuse to talk this way, insist that one must have an experience to be 'aware of'

the duck, But as Chalmers points out, this difference is largely verbal (27).

.....................................

As to defining zombies, a 'zombie' is a physical system identically similar to a human being physically, but without experiences.

It's easy to conceive of physical systems without experiences. For instance,

 I believe my desk has no experiences. I understand perfectly well what that proposition asserts.

I can conceive of its being the case.

So there seems to be no problem here. Of course I cannot imagine what it is like to BE a desk without experiences--that involves imagining what it is like to be a thing

such that there is nothing it's like to be it! But I don't have to do that to conceive of my desk not having experiences. I know what experiences are from my own case, e.g.

the experience I have when I bang my thumb with a hammer, the experience I have when I taste a mango, the experience I have when I see red, and so on.

To conceive of the desk not having experiences is to conceive of the state of affairs that the desk just doesn't have any states like these.

To conceive of a zombie is to conceive of somebody physically just like me who has no states like these.

Of course I cannot give definitions

of states like 'the experience of seeing red,' these are ineffable, one has to have them. This is simple empiricism. Definition must end somewhere:

it ends in experiences ('Impressions'). Still I know from having them what these experiences are, and so I know what it means to claim that desks,

rocks and zombies don't have them. That is, I know well enough what they are said not to have.

So I know what zombies are supposed to be, no deep problem about describing them: my zombie twin is a physical system physically just

like me but without experiences. But whether zombies really are possible remains to be seen.