"Is it true that the current Yahoo Community Guidelines are broad enough that almost any question or answer may be reported as abuse?"
No, I don't believe it is. The Community Guidelines basically say no spamming, no flaming, no advertising, and basic stuff like that, all of which are pretty clear. The only one that's hard to interpret would be what classifies something as "chat" (especially since we have an entire section dedicated to polls and surveys), but if you really want to, even that can be avoided.
"What's your experience of Y!A been? What questions or answers that you consider inoffensive have been removed? "
Way too many to count have been wrongly removed. I had my first account suspended because of it. So overall, I'd say my experience has much room for improvement.
"Does Y!A (and by extension, us) have a duty of care to censor questions and answers? Or, does it (and we) have a responsibility and right to let people say whatever they want even if it is morally repugnant?"
Neither. Concerning the first topic, just because they use us as their outlet doesn't mean it suddenly becomes OUR responsibility. If we see something offensive, we have every right to report it, but you shouldn't feel obligated to do so. I mean, it is Yahoo's website after all. If they really wanted to buckle down on violations here, they'd hire some people to do it on their behalf, not rely on the community. Concerning the second topic, this is a privately-owned website and as such the owners of it made their own rules. It just so happens freedom of speech isn't something they believe very strongly in.
"Also, what is your opinion, please, of the reporting system?"
Horrible. The reporting system here is probably one of the most inefficient and broken pieces of crap I've ever seen.