Don't know if anyone on here does law/eng. I personally never considered eng due to my weak maths/science abilities (ended up choosing commerce/law), but it's not that uncommon as a law double, probably the 4th most common one after arts/commerce/science.
Couldn't tell you much in terms of degree outcomes...I assume most end up going down one track or the other (i.e. being admitted to practice, or becoming an engineer), although I've known one or two eng/law students who were aiming to get work in relation to construction law/disputes. That's probably the area where the degrees really complement each other, not that degrees need to complement each other, by any means. In terms of commerce - have you thought much about which majors may interest you?
As for the six subjects thing - I highly doubt it, the faculty would generally be unlikely to let a first year overload and do five subjects, let alone six. Not sure how the prerequities thing would work, to be honest, but you'd likely only do the 4 units.