1. I don't get this obsession with one-seat rides. The benefits aren't so great that it's worth what you lose. Depending on location as well, good luck building a grade separated junction.
2/3. Lots of options. Divert around downtown for 3-4 years, split routes, etc. You could even use Adelaide.
4. This is just plain stupid. This is basically the equivalent of doubling the amount of stations you'd need downtown and doubling construction and maintenance costs.
5. Shallow. There's no chance for this downtown. It's too built up and correct me if I'm wrong but this type of tunnel is usually associated with cut and cover construction?
6. Utilize how? It's not exactly problem free, either for an elevated line or if you want to build an underground station anywhere under the rails.
The link should really be read since it does answer a number of questions in great detail. Here are my brief responses.
1. One-seat rides have the potential to reduce the bottleneck at Yonge-Bloor (and at Pape, which was not designed to be a major transfer point). Instead of riders all transferring at one station (or two), riders would be spread out across the system and would wait one extra train to be able to get downtown. This could reduce headways and improve system performance. Even if you ignore these benefits of the one-seat trip, there is also the political issue. Aside from taking up space on the street, what hurt the Sheppard LRT the most was the number of transfers needed to get downtown. In order for a DRL to get widespread support across Toronto, you need support from the old Boroughs, since they outnumber the old City. These people want a one-seat trip.
2. Although there are other options to help maintain service during construction, the existing streetcars are a less expensive and a better capacity alternative than using busses on Adelaide or diverting streetcars or supporting streetcars overtop of excavations.
3. Although for the final location Queen or King is acceptable, the Richmond/Adelaide alignment allows for some of these other benefits to be realized (i.e. less construction disruption, and at Queen the Yonge subway is not deep enough for the DRL to go over top of it)
4. The proposal has smaller stations since they are at a shallower depth and each station is for one direction only. These two “half stations” would have tunnels connecting between Richmond and Adelaide. The proposal talks about some double track stations, which I think may be overkill since double platform (access through doors on both side) would be sufficient for the busy stations – still better than anything on the current system.
5. In terms of PATH, I believe that there were only 2 or 3 conflicts that must be relocated. In terms of other utilities, there may be some conflicts but since there is only one direction on each street there is still ample room in the right-of-way to relocate these. For construction, the proposal recommends “Icos-Veder” or “cover and cut”, which essentially digs trenches, builds vertical walls and roof, digs out soil and builds floor. The excavation is no wider than it needs to be. This is better suited for shallower excavations. Using deeper tunnel boring (TBM), the stations are still typically built with open cut (but stations are much deeper and the excavation is much larger). In downtown it is nearly continuous anyways since stations are 150m long and at +/- 600m spacing.
6. Utilizing the railway for deeper tunnel boring is proposed to bring the DRL back up to B-D. There are typically few utilities under the railway fight-of-way so there should be minimal conflicts. It is true that stations are more complex under railways, but since this portions is away from downtown, there would only be about 3 or 4 stations built on this stretch.