Instead of going deep, I wonder if we can go shallow. Can the DRL go over the Yonge line. Just by walking the stairs, I estimated that the distance from Yonge track to pavement level is about 7.0m to 7.5m at Queen. A subway height is about 3.7m and a Mark III metro (for the DRL) is about 3.3m. Probably the thinnest support slab between subway levels is 400mm, and between subway and street another 400mm. Thus, about 7.8m is required to achieve this. Was my crude measurement off enough that the actual height is more? Does this value increase as you go south and reach Richmond, or Adelaide? Of course under street utilities and storm sewers may be a problem and require a cost to relocate.
But the advantages of this would be huge. Construction would be much faster to excavate only 4m to 8m deep stations through downtown instead of 20+m - and much cheaper too. Platforms closer to the surface are more desirable since less time is lost from street to train. Ventilation costs, dewatering, emergency exits, etc. would all be lower cost. It may also be possible to bridge over the Don River just south of Eastern Ave instead of a deep tunnel under.
I also wonder if the "Mezzanine" could be built below the station. You would go down 1 level to access 1 platform, or down 2 levels to the "Mezzanine" and then cross over to the other side platform (or centre platform if 3 platforms are used) from below. This somewhat deeper excavation (still less deep than a station with TBM tunneling) would only be required for a single point (or two) at a station, and not the entire length of the station box. The key to this DRL is to keep the costs as low as possible and build (or at least announce) the full DRL long as being a priority. That would achieve the maximum support from all of Toronto.