I think we've managed to cover that both formal and notional agreement approaches seem to have gained acceptance. However, your later example is hard for me to follow. This does not seem to be a case of applying notional agreement.
Which (sentence) of the following sentences is considered correct in Australia? (grammatically correct)
Which (sentences) of the following sentences are considered correct in Australia? (also grammatically correct)
In other words, determining whether to use singular or plural is dependent on the subject whether visible in the sentence or not. In these examples you should not be using the visible word "sentences" to determine agreement because it is not the subject.