Hell, if it truly is impossible to do anything with Milton Line and CP, then we should get creative and look at things like a branch off the Lakeshore line, even if it has to be 100% elevated. The status quo is unacceptable, and the dumbest solution to this would be to throw $10B at a Line 2 extension that still leaves most of northern Mississauga and all of Milton (upwards of half a million people) with severely inadequate service.
By the way, I am all for truly HOV (HO as in buses, not 2 pax cars) lanes on all highways. I think GO buses would become pretty popular indeed if they could actually travel at highway speeds on peak. Restricting HOV lanes to buses would help a lot with lollygaggers and congestion caused by cars merging in and out of the HOV lanes. I could see it helping quite a bit with point-to-point GO routes, connecting lines circumferentially or between suburban nodes.