There does not necessarily have to be a tunnel if there is a bridge over the tracks.
There is already an adjacent underpass next to Confederation station. Theoretically, Metrolinx can add stairs to both sides of the underpass.
Purple = The bridge already has room for an island platform.
Yellow = Stairs can be installed to allow people to go to platform on opposite side
(Photo credit: me,
taken from this post)
(The yellow rectangles would be on opposite side, but you get the alignment)
The low-cost method would be
(A) Pour concrete in the
purple platform-reserved area (
north platform, but further to the east of bridge rather than on bridge), as well as on the south side (
south platform, south of southernmost track)
(B) Put a temporary sidewalk across unbuilt tracks towards the actual northernmost platform (since no tracks in the way will exist for now). For now.
(C) Install stairs in the equivalent
yellow areas (but behind-the-photographer edge, not the pictured edge) for accessing south patlform.
In theory, only $1M-2M to build the barebones interim Confederation station.
In theory.
Or only one platform may be needed -- e.g. north platform -- if CN can let GO trains mainly use the north track for both directions -- then no stairs would be needed.