Addressing your two points here, look at the image below:
1. The plan does not have property laid out to do a straight up full-width staircase (property in red). Obviously the stairs are planned to be east-west. However the width does appear to be similar to the southern entrance, rather than the narrower platform size that currently is in use.
2. Also note that to the west there is a plan for a second tunnel (in orange). This includes a second "plaza" open space leading to the new tunnel just as it currently exists in the central location.
3. The only uncertainty is the eastern third tunnel, which is contemplated to connnect directly to the streetcar loop / future LRT extension directly.
Nowhere else in the GTA is Metrolinx engaged in strictly pedestrian access. Any improvements external to the core transit (Rapid Transit or Regional Express Rail) mandate are tangential. But obviously it is being considered as part of the Exhibition Station upgrade.