What could have made the Invasion of Quebec 1775 successful?
Definitely not David Wooster, I think if Richard Montgomery or John Thomas had lived they would have been the commanders able to secure a victory over Quebec City. The keys to the initial victory over Quebec are cohesion, initiative, treating the local inhabitants fairly, and an intensive diplomatic effort with the Iroquois.
Clearly Arnold’s march to Quebec from Maine was a failure. What I propose is for an early effort to be made to secure Halifax. This cannot be done without earlier and greater French support. Supposing Montgomery captured Quebec, and supposing the Thomas captured Halifax they will still be driven out by the much more numerous British army. I think the British pincer move from the Saint Lawrence and the Hudson will still play out, but fail harder than it did in the OTL.
Assuming the best for the Patriots, they will have prepared fighting positions which allow them to pull back into the strategic depths of Acadia for Halifax, and upstate New York for Quebec. Thomas will make the British fight dearly to obtain a base in Halifax, and then escape back to Maine. Burgoyne’s army will attack Quebec City, then be bloodily lead Into Schuyler’s trap by a withdrawing Montgomery, and defeated by a waiting Arnold. It is possible for the American withdraw from Quebec to be so bloody for the British that this field army is defeated at Fort Ticonderoga rather than Saratoga.
Assuming the early French support could only ever be more volunteers, money, and equipment (instead of a field army or the actual navy) the British will still take New York City (and New Port, Rhode Island). I think the taking of New York City could be made more difficult for the British, and that they could be locked within the city. Howe will still attempt his Chesapeake Landing, which presents an opportunity for him to be captured (maybe at Brandywine).
With at least Carleton, Burgoyne, Howe, Cornwallis, and Knyphausen captured that leaves Clinton in New York and whomever in Quebec City and Halifax. The Patriots could attempt to reinvade Canada, but I’m not sure it’s necessary because the people in Canada and Nova Scotia will be unhappy with the harsh British occupation compared to the light Patriot occupation.
In this best possible scenario (for the Patriots) the British will quickly negotiate a peace that would turn over all of British North America to the Patriots. I don’t know just what combination of factors is needed to make this happen, and I think it is the very extent of plausibility.