Yes, specialist is definitely helpful for actuarial studies. It's good for any course that requires a high level of math, so definitely take it up if you're interested in taking something like that. As I said above, you should take it as it's a prerequisite VCE subject for some math units in uni. So if you haven't done it then you have to do a lower level math unit which will probs be the equivalent of spesh.
Well, from what I've read and been told it's much easier to go from an engineering based role/career into a finance/commerce one than it is from finance/comm to engineering simply because the quantitative skills you gain in engineering can be applied to finance/comm moreso than the other way around. To add to that, completing a 4 year B.Eng with 1 year experience would probably make you more appealing then a fresh 5 year B.Eng/B.Comm graduate, and again you'll probably learn some of the financial, accounting and managerial aspects while you work as an engineer although that may take some time to develop. Engineering is a great degree to have since you can work in multiple industries with the skills you develop and learn from it, it's not restricted to the traditional engineering roles.
It's tough to get a 'comfortable and enjoyable job' commerce jobs these days especially as a graduate. There are tons of graduates applying for the same positions, you'll need a decent average 70+ to be competitive, good extracurriculars and relevant work experience - and knowing people helps a lot. If you're lucky enough to get a grad position, I don't think life would be so swell either. Depending on where you work you'll still be studying for CPA/CA (acct) or CFA (finance) exams while you work, you'll probably have to stay at the office or complete work outside of work hours and possibly work Saturdays, network, suck up to the boss etc. Of course once all this is done then you should be fairly comfortable although again it does depend on your job, role and your responsibilities.