Can't see how it could make a difference to your chances of getting into law and certain types of engineering.
Could affect your chances of getting into dentistry 'cause you'll need the GAMSAT, and from what I hear chemistry makes up a lot of section 3, and it's pretty high level chem too
You could always take fundamental chemistry at uni, and then move on to the higher level stuff and ultimately be on the same level as all other chem students by the start of the 2nd year's first semester
Self learning for the GAMSAT is also an option, but that might be pretty hard for the chem portion cause it goes into great depth from what I hear
You'll also need to complete first year Chemistry at uni if you want to get into the DDS (but it may only be Melbourne uni that requires you to complete certain subjects if you want to do the DDS - at least that's how it is for the MD anyway- every other uni may only require you to do an undergrad degree of any kind without the need to satisfy prerequisite subjects)
I guess you could always just go for undergrad dentistry. Pretty sure La Trobe only cares about your ATAR when offering positions to their dentistry course - it doesn't even consider the UMAT