Nowhere does it say that you are going to be asked for two related texts. You need to be prepared to answer whatever question they give you with however many texts they ask for, whether it be two or only one.
If your reasoning is correct, why do the Board of Studies even bother specifying the number of texts in the question? A set text and ONE related text is still technically a variety of texts, is it not?
No, because although you wouldn't get marked down on having more texts than they've asked for, they're only going to mark one of those texts, and because you divided your writing time between 3 texts, there won't be nearly enough detail in only one of them to get decent marks, unless you're amazing.
You tell me...
Likewise, why would you think you had to write a story about flowers, when it's obviously a metaphor? It's people like this that ensure everyone else gets a slightly higher ATAR =D
Yeah, that's exactly how it works. Whatever you write that's correct, you get marked on. If there's anything there that's wrong you don't lose marks, they just ignore it. That goes for Maths and English.