Just my opinion (having been in your position years ago): Neither option really fits the criteria of a 'cost sharing' flight but unless you bend the aircraft in a high profile manner I doubt anyone is going to notice or care.
The later case you mention with the other contractor could get tricky of you have an accident and that contractor goes after compensation from someone as the someone will probably be yourself.
Legitamicy aside, I've completed such flights as a PPL years ago and I personally wouldnt worry about CASA, as you have already found they cant even give you an answer themselves. Just consider your own liability if there was to be an accident.