It's hard to say - I was always a 1000 word kinda bench mark but both my trial and my HSC essays were wildly different (trial longer - HSC shorter) so it really depends on the question!
Yes, you're right! In an essay, perhaps you can use ethical issues of IVF, euthanasia, etc, to support how a Christian tries to "do good" in accordance with the ethical teachings about the sanctity of life and so on.
Yes! This works! I do admire how much attention you're giving this but I'm worried it's worrying you! Any time Paul refers to God, or the transcendental being, you could say he's referencing the trinity. But I would focus more on how Paul inspired the Church to grow and expand and develop, which has obviously been so strong thanks to the principal/core beliefs of the religion.
It really depends on the question - there's huge flexibility in how to approach this kind of response. Scroll down to page 18 of
this same answer and see the huge range of things that you could cover, and the order you could cover it in. I would follow a structure similar to what you've suggested above, but with some flexibility for also engaging with the principal beliefs and sacred texts concurrently with the others instead of just separately. In Christianity, world peace starts with how one achieves inner peace, or personal peace, and then expands through the community and the world - so it's not inappropriate to discuss inner peace but also not compulsory. Christians seek peace through the individual and the community.