This is pretty much what I expected. Quae refers to everything Helenus has said, i.e. Praestat ... Troiam ad caelum tolle! Such statements require the neutral gender (due to the lack of an obvious masculine or feminine antecedent), and the complexity of the statement causes the plural, ergo: quae.
Compare the stock phrase Quae cum ita sint...
Sperate miseri, cavete felices.