It seems like Toronto-Buffalo rail service will never be all that convenient unless the customs/border checks are on the Buffalo side.
Cross-border services will only be convenient with the US if the border formalities can be exacuted at either the departure or arrival station and easy cases (NEXUS holders or US/Canadian citizens) can be prioritized to minimize the delays they experience. Same reason why the border facility for the Toronto-Chicago corridor should be in Detroit and not Windsor, because more people will travel from Detroit towards Toronto than from Windsor (much smaller) towards Chicago (much further), which would also allow to extend all four TRTO-WDON trains to Detroit Michigan Central, rather than extending just a single CHI-DET traint to Windsor.
I wonder if it could be designed to hold of a negative decision until after the 2029 election
A decision will be need to be made by the next government before the 2029 election about the fate of Phase 1, but thankfully that is the by-far shortest and cheapest segment (MTRL-OTTW), which also happens to be the one where the cost and travel time difference between true HSR and Higher-Speed (or even conventional speeds) is the least significant. Any decision regarding Phase 2 (OTTW-TRTO) can thankfully be deferred until beyond the 2029 election…
With your convincing argument about the difficulty in servicing Quebec City out of Central Station, I wonder what would make sense is running from Ottawa to Quebec City, with a stop at Dorval (meeting the REM), onto the CP track through CSL and stop at Canora in TMR to meet the REM again, and provide good connections to downtown. A stop at Parc station (maybe a stop at de la Concorde metro?) and then turn onto the Trois-Rivieres sub. With limited services terminating in Montreal just stopping in Dorval and Central. All trains stop in Dorval, and it be the main transfer point (probably would help if they had every 15-minute service on the Dorion EXO line).
Obviously it's flawed - but is it more flawed than trying to serve Quebec City via St. Henri, Ville St. Pierre, Lachine, the CN St. Laurent sub, and however you get it to the Trois-Riveries sub from there.
(another way to save 10 minutes is to go into Lucien l'Allier instead of Central - would be easier to serve Quebec City from there too).
The three challenges of extending HxR from MTRL to QBEC are in that order:
1) Not inconvenience travellers from MTRL towards OTTW/TRTO.
2) To find a convenient way to transfer between QM and MOT trains.
3) To find a convenient downtown(-ish) station for QBEC-MTRL trains.
Challenge #1 prescribes the Status Quo (Terminus at Gare Centrale, Satellite station at Dorval Airport).
Challenge #2 prescribes Dorval as transfer station (as Gare Centrale would escalate trave times).
Challenge #3 leaves us with only two solutions: either extend QM trains beyond Dorval to Gare Centrale (which would not be time-competitive with transfering to the orange line at De La Concorde) or to have a “downtown” station at either Parc (which lacks rail links towards downtown), Canora (which lacks space for proper intercity rail station, especially one which acts as downtown station) or Namur (which would have an inconveniently long walk to the orange line).