Toronto should drop all talk of it's stupid tunnelled DRL plan and put all resources towards the ST subway. It can start construction immediately and that is the only way it will get any money from the feds.
What you say about ST is complete conjecture. We don't know what ST will entail, so we can't say when it can begin construction. What we do know is that the Relief Line and Scarborough Subway should be construction ready by 2019.
There are many factors that could delay components of the ST plan, deepening on its scope:
1. City planning is looking at redesigning the ECLRT west extension. Any significant modifications will require new design work. This component likely won't start before Scarborough Subway or Relief Line.
2. Locations for additional GO RER stations haven't even been selected yet. These will all need to be designed from scratch.
3. Any modifications to Union Station will require a ton of design work.
4. Upgrades to Main Street station, again, will require new design work
5. Modifications might need to be made to facilitate 10 minute frequencies at Kennedy and Mt. Dennis Station. EA, design stage, expropriation will all take time.
6. You seem to want ST to have its own fleet of cars. That might require a new maintenance/storage facility.
Cash flow might delay construction as well, as we've seen with Eglinton, Finch, Sheppard LRTs.