Actually I think you're failing to understand both the complexity of the existing canals built in Canada, and the difference between the Erie Canal and a proposed St. Lawrence canal. Principally that the canals built historically by the 1860s (Lachine, Buhearnois, Rideau) were greater in depth that the Erie Canal ever was, and these can all use the St. Lawrence River rather than having to depend purely on a canal project that has to go over rivers. What is suggested here is something that will allow steamers and river boats to shoot the St. Lawrence with ease compared to the difficult runs of OTL and allowing ships in excess of 500 tons to transit the canals. OTL that was possible by the 1850s, so maybe by the 1860s-80s you can expand them to accommodate ships in excess of 1000 tons. Nothing about the proposed "seaway" is outrageous for the technology of the time. We're not talking about the 10,000+ ton cargo ships of today, just smaller ocean going clippers or steamers. Even then most of those will stop at Montreal/Halifax then move their cargoes to river boats. This isn't impossible for the technology of the period.