Well as far as it's relevant to Manchester Exchange I think it's worth continuing here.
The proposed HS2 route in tunnel to Ardwick was basically chosen because they decided that Piccadilly was the best terminus site. That makes sense when you consider how many more rail or Metrolink connections can be made there than anywhere else (and even more under Northern Hub). There is also space available for the new station alongside the existing one.
If you look back through the documents on their website you will find a family of options that approached Manchester through Eccles. These would have linked to one of two shortlisted terminus sites west of Salford Central. These were discarded mainly on the grounds of poorer city centre access, no Metrolink and worse rail connections.
Without going back through the documents I can't remember if Exchange was considered too but it probably was. However there is no spare land here at ground level (or at least not enough to create platforms 400m long). The only option here would have been to deck over the site and put a new station above the existing tracks. As well as being very expensive this would have been visually obtrusive, overshadowing the centre of Salford even more than the existing viaduct does, especially as the approach tracks would have had to remain at a similar level over Salford Central before descending further west. With the possibility of linking to Victoria the rail and Metrolink connections would have been better than at Salford Central but still worse than Piccadilly.