Is having my exam remarked even worth it?
Most of the time, it isn't. Your ATAR is a
tertiary admissions rank, that's it. Once you get into a course, it's irrelevant 99% of the time. In my opinion I think you should put all your effort into the first semester of university and request a transfer. Your university scores outweigh your ATAR and are more relevant to your actual achievements (because the marks are usually based on merit, rather than ranking/scaling).
I view it as being completely irrelevant after you get into a university, because simply being enrolled at a university creates a pathway for you to transfer later on. But since you've already requested it, you may as well get it inspected.
Should I hire a tutor or something to inspect the exam with me? (my HHD teacher left, and the ones that remain have no experience)
If you feel like this is a good idea, why not? Usually as the student, you can detect whether something hasn't been marked adequately. But it's always good to get someone else to confirm your doubts!
Can an exam remark reduce your score?
As Alter said, it cannot. This is the same process that occurs in university remarking as well. If you request a remark and the remark results in a higher score than what was previously reported, then you will get the highest one of the two.
Good luck