You seem to be in a very similar position to me last year.
I was super keen on Eng/Law right up until the Change of Preference expo at Monash.
Hours for Eng/Law in the first year are nowhere near 24 hours. An engineering first year subject is 4-6 hours, and a law subject is about 3-4 hours per week as I understand. So you end up with 14-20 hours/week depending on what subject you do.
The reason I chose commerce/engineering over engineering/law, is because I talked to quite a few people + did some research/went to change of pref expo. There was an overwhelming response to how useful a commerce degree is in an Engineering field (and vice versa), but when I asked about law it was always just something like 'it's always good to have a law degree'. There are some suspect jobs which use both, patent law, consulting (though more commerce related if anything), and the legal side of an engineering firm, but I don't personally think that doing an extra year of law, for the loss of the sheer broadness of a commerce degree and the gain of not too much is worth it.
Yes, Engineering/Law sounds really cool, and Comm/Eng is not something that will make people wow when they ask you what you're doing, but I really don't think Eng/Law is anywhere near as valuable as Law/Eng.
Additionally Law has significantly dragged down the GPA of some of my very clever colleagues, which does have an impact on their employability at the end of their degree.
In all honesty, I think if you get involved with the right things, you can be successful in either double, but in my opinion from the outset Commerce/Engineering is the way to go.
If you want to be a solicitor/barrister, I'd recommend ditching Engineering right now, there is no point wasting time with it if that's definitely what you want to do.
I personally never did any Commerce in school (nor did I do law), but I don't think it's very hard to pick up if you've done well in methods, maybe year 10 humanities (high school geography is the right way of thinking for some of the core subjects), and if you're decent at English. That said, I wouldn't say you'd be at a huge disadvantage having not done any comm subjects.