I think you get the applicable taxes and the fare portion for the return. That may or may not be about half though, since the outbound and return portions won’t necessarily have been the same. However, it probably won’t be far off because the fare paid is about as low as they go so it’s pretty unlikely there was a serious imbalance between the two halves.

I don’t think there’s any risk of being reassessed at a one-way fare on an involuntary change.