That's some sloppy writing. Do not use it as a model.
1. What changed was your initial conception. The writer neglected to actually put that antecedent on the page, making the word "this" weak and flabby.
2. "As it were" belongs after "tried in the fire". I believe that the writer meant it that way, too, but carelessly stuck it in the wrong place. There is no actual fire, so "as
I'm sorry I made some mistakes. The correct one here:
You may have begun your essay with a strong opinion, or with only a scattering of ideas, but as you read and thought this changed. The scattered ideas became more organised and coherent, the strong opinion altered - perhaps a little, perhaps a lot. It has, as it were, been tried in the fire and it is better and more
1. What does 'this' refer to? It refers to "your essay's a strong opinion, with only a scattering of ideas" 2. What does 'it' refer to? It refers to "being tried in the fire." (meaning being subject to a lot of criticism and debate.) 3. What does the last 'it' refer to? It refers to the opinion stated in the essay.
No. There is a definite antecedent. Compare with: It is raining. It is 5:00 already! (dummy it - We have not idea what "it" refers to.) It is proper protocol to curtsey before the queen. (anticipatory it - refers to a phrase that comes later in the sentence.)