Mr. Goguen's comments would be our view as well. It was drafted to take into account that we start off from the premise that you're not guilty of an offence if these things are established, but that if those factors weren't established, the act you have committed may well be an offence, so to add in “would otherwise” be an offence adds a bit of an element of confusion.
That said, we know that at the end of the day, when the judge comes to the determination that your otherwise offensive act was done in defence of property or, in other cases, in defence of self, it's not going to be an offence. It's true that at the end of the day it would have otherwise been an offence, but as the law is drafted, it is preferable to refer to “the act that constitutes the offence”, because the starting point is that your act is an offence except that it was done in self-defence or in defence of property.