Correct, sharing tracks won't impact most operating costs though I suppose you can split corridor maintenance costs between more runs.
Provided HSR ticketing is available through the online airfare booking systems like Expedia as a connection option (Hotwire, Expedia, etc.) and perhaps even get Air Canada/WestJet on board as a partner, it'll be fine.
London to Toronto flights are not profitable (not a loss either). They exist purely because they connect to a profitable flight. Kitchener to Toronto or Pearson to Union flights don't exist at all, so Air Canada or WestJet selling those tickets would be pure profit from the commission earned (usually 10% IIRC). Several European trains are bookable as connections from travel sites as flight connection option. By serving Pearson this becomes an option. Murry hints at this possibility by calling the new Pearson train stop Terminal 2.
Again, tons of open questions including business associations but there is enough of a business case to do an EA.