DON'T DO IT!!!!!!
The marker's correct (well, he would have been in any case: markers are ALWAYS *cough* correct). Related texts are supposed to show how well you understand the topic by applying it to other texts. If you just use the prescribed texts you're not really showing you know anything, other than your inability to find related texts.
You've also mentioned the two prose fiction texts on the prescribed texts list. What text is your prescribed text? You shouldn't use two texts of the same type (prose fiction, poetry, film, etc) as related texts because that limits your ability to write widely; also, if they ask you specifically for a visual text you'll be stumped. I don't know if they can do that, but they did for our trial...
And if you were a marker, and you'd just read fifty thousand essays on The Shipping News, and then some person writes about it as a related text they'd be pulling their hair out. Besides, they already know it all too well - they've just read a bunch of essays on it - so you can get away with less.
In this case don't listen to your teacher. You're allowed to use prescribed texts as related texts, but they advise against it, and specifically NOT ones from YOUR elective.
Sorry if that sounds a bit harsh, I get scared when people say things like that - why bother risking a good mark?
