"This" is deictic: it identifies something by pointing at it. The pointing might not be physically extending a finger, it might just be looking at the thing, or even more abstract; but in principle your attention is being directed at the thing.
"It" is resumptive: it only makes sense if both parties in the conversation already know what is being talked about.
So your first dialogue is completely normal.
In your second dialogue, the first answer "No, this is not a gull" is possible, and would tend to imply that the answerer is at pains to identify this, as opposed to other things around. "It" would also have been possible.
In the rest of the dialogue, "It" would be normal, and the use of "this" would tend to suggest that the questioner is pointing at a different object each time. It is not impossible with your intended meaning, but it is unusual, and would suggest to me that there is something rather formal or artificial about the conversation. (I don't mean "formal" in the sense of "high society", but in the sense of "something very structured, almost ritual").