Hey Barcelona10,
The reasoning in the stimulus is rather convoluted, which may be why you're having trouble with it. Let's simplify the argument:
- Premise 1: We believe some poems express contradictory ideas.
Premise 2: No author wants his poems to communicate contradictory ideas.
Conclusion: A poem does not always mean whatever the author wants it to mean.
On its surface, this is a pretty solid argument, provided that the contradictory ideas in question were part of the meaning of the poem. There is a slight disconnect here between "ideas" and "meaning": and that's the crux of the issue. What if the contradictory ideas we believe are expressed by the poem are not, in fact, part of the poem's meaning? This is the logical opposite of answer choice (E), and--if true--it completely destroys the argument. It does so by showing that our belief in whatever a poem expresses may not have anything to do with the real meaning of the poem. If so, then the meaning of the poem could still be whatever the author intends to communicate by means of the poem.
Admittedly, this is a rather abstract and convoluted argument. But you greatly increase your chances of getting the question right by 1) simplifying the argument whenever possible; and 2) apply the relevant technique in order to differentiate between Contenders and Losers. In this case, this was the Assumption Negation Technique.
To answer your last question, personally I wouldn't diagram this stimulus: although there are elements of conditional reasoning in it, you risk making the argument even more convoluted than it already is
Hope this helps! Let me know...