Hi,
1. I found that my marks went down slightly. That's because in unit 3/4, the teachers made the assessments harder than vaca exam level but unit 1/2 tests and exams were pretty easy. But, after that, I put in more effort and my marks improved dramatically.
2. Sorry, but I can't give you a definite answer because it depends on the difficulty of your school's 1/2 tests. Also, your 1/2 marks don't always reflect the study score you'll end up with. Just put in the effort and do your best.
3. First of all, doing more practice application questions will expose you to different questions. Whenever I see an application question, I look at the equation that is usually given and work out what that equation is telling me. What are the maximum and minimum x and y values? What do they mean? What do they show me? If I derive or antidiff it, what do I get?