What highway are they planning on building over either of those stations, on either of the planned alignments? The only highway located close to a station would be at the Exhibition Station on the Ontario Line.
Daily overcrowding? Please. The area is busy and getting busier, but let's not conflate the reality with the area as it sits now and with what is likely to happen in the near future with any fantasy visions of what it might become in some sort of idealistic world.
As for Regent Park, it's always going to be a long walk from the bulk of neighbourhood no matter when you situate the station. Siting the station at Parliament or at Sumach isn't really going to change things greatly.