Going down through Lake Champlain is doable and pleasant just longer. We went to Bermuda from Lake Ontario in the spring of 2007 and the Erie was closed by late spring rains so we went down the St Lawrence to Sorel. The distance from Whitby to Albany was twice as far as going through Oswego but you have substantial current in your favour in the St Lawrence.
There is a do it yourself crane at Sorel for mast removal and an even better setup at the Castleton Boat Club just south of Albany for putting it back up. I would call and make sure it was not damaged.
I would not suggest going down the St Lawrence. It is very long way and in the lower St Lawrence could be cold. From Nova Scotia eastward you tend to be fighting the prevailing winds. Someone mentioned Halifax to Bermuda to Virgin Islands - you would want a really good weather window for the first part.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.