I think the tunnel option could actually end up cheaper and more practical than the surface option because:
a) running the tram on the east side (Ottawa-bound side) of the Portage Bridge would mean that it will be independent of the traffic light cycles on the Laurier/Maisonneuve intersection. A surface tram on the north side of Wellington would have to somehow traverse traffic, which would impact the flow on the bridge more.
b) Catenary-supplied power in a tunnel won't need more expensive battery powered trams as suggested to avoid overhead wires along Wellington. Add to that the likelihood the NCC will want solid granite pavers between the tracks and bespoke structures that don't clash with heritage buildings.
I don't think the Sparks tunnel should go beyond Lyon, which would mean a mere 500m cut-and-cover tunnel. If there was a head house on Sparks, the rest of the CBD is totally accessible from there along a perfectly walkable pedestrian mall.
Rideau Wellington Transit Mall? Been there. Done that. Failed. Considering the fact that there is zero chance most of the buildings on Wellington will have commercial retail or active public function, it could end up being a creepy windswept space after hours