This is actually one of the subplots from one of my timelines.
-After the 7 Years War, most of Quebec's population is expelled, Acadian style. New Englanders emigrate up and Quebec joins the rebellion.
IDK, New England already had serious issues with LOCs with Canada. Also, the development level of Canada at the time was more at the level of 1690, not 1775. One huge reason why the Canadians weren't interested in independence.
-With Nova Scotia also in rebellion, the British wind up in same position in 1781 as OTL, holding New York but no Canada (except Newfoundland).
Nova Scotia, Prince Edward Island, and Labrador are impossible to hold (the British MUST have Halifax to conduct any kind of war against the Rebels) against an enemy with naval supremacy. New Brunswick is another matter.
- Lacking any place to send 60,000+ (probably many more), they demand in the peace a "reservation" for loyalists, naming the Tri-Islands of Manhattan, Long and Staten Islands (and Newfoundland).
Loyalists were sent to the UK too.
New York City is the living heart of the United States of America, and were it not for American sectionalism would have always been and now today our national capital. Such a "reservation" would never be agreed to by the Second Continental Congress, and in this even France and Spain would agree. For the Spanish the precedent of Gibraltar and the Balearics, and for the French the memories were too fresh even in the Middle Ages of Calais and Aquitaine.
-France, Spain and Netherlands feeling (rightly) that the US has won enough and can live without a few islands. They aren't going to fight for another year in hopes of getting the colonists one tenth of 1% more land.
Couldn't disagree more. How much of London (or Southampton), Paris (or Brest), and Madrid (or Cadiz) represent a "territorial percentage" of their respective countries? Oh, but WHAT a strategic percentage. Pre-Age of Steam/Railroads, you are effectively bisecting the USA between its then most populous region (New England) and the rest of the country. Reads like an "Ulster Solution".
If the British are strong enough to get a deal like this, then they'll keep on fighting anyway.
-Constantly harassed by Loyalists in New York to reconquer America, Britain does an early version of the 1812 "Impressments", halting trade on the Hudson, etc, etc, etc, basically keeping the Americans mad.
Well, keeping in the spirit of this "mad scenario", one would wonder what Britain loses in the outside world holding on to this "1/10th of 1% land". Britain loses the Spanish Campaign?

Or at least never achieves its mission of being a bleeding sore for the French? And the USA certainly ITTL won't have a miserable militia force as OTL.
- Eventually a war breaks out, probably in alliance with Revolutionary France.
You mean Napoleonic France? Please don't say "butterfly". That's too easy a fallback when dealing with ATLs and The Corsican.
I compare the Tri-Islands of New York less with Gibraltar than with Hong Kong. Eventually American would take those islands, it is only a matter of time.
Agreed. Incredibly.
Probably during the Napoleonic Wars. With Britain facing the danger of invasion (forget 20-20 hindsight people!) there's no way Parliament will allow such a running sore to continue (defending a City-State 3000 miles from home without Gibraltar's natural defenses?).
Delays the Loyalist Exodus until the earliest years of the first decade of the 1800s, but that's it. Not if Britain wants to fight Nappy everywhere else in the world. Its not like the British have a huge army to fight with at this time in the Napoleonic Wars.
Okay, in face of all this, what is a more probable place to attack, excluding the south (lost to the Tories)?
The Tories alone had as much of a chance of taking the Southern American Colonies as the Patriots had of taking Quebec. Ferguson did a spectacular job of raising American Tory support in the South, that's true. But even if he didn't have Banastre Tarleton cutting his legs off professionally at every opportunity, then some other "fire & sword solution" senior officer would have filled in that natural gap of strategic/operational thinking instead.
Or I am misunderstanding your meaning here?
The reason Washington didn't attack NYC wasn't because he didn't want to, because he certainly did. but he lacked the support of Rochambeau and some subordinates. Washington was determined to take NYC, if I recall correctly he was initially reluctant to move south to Yorktown.
When it became obvious to Washington that the French would not support a campaign against NYC, he asked for their support for a campaign against Charleston and Savannah. Problem: Charleston could always be taken back by the British once they gained naval supremacy (while Virginia and North Carolina and the interiors of SC and GA could not). In the Age of Sail and no railroads, Savannah is useless with an enemy controlling Charleston.
The French knew this, and made it clear to Washington that they HAD to reinforce the West Indies immediately. Yorktown's victory was due to the total miscalculation by Graves and Clinton that the French would never strip the defenses of the Caribbean for one day.