Dunkirk is a small harbour
You keep missing the point : giving an harbour, no matter how small, is an important strategical issue when it comes to control the coast as such harbour becomes sort of revolving door on it for anyone controlling it and their allies.
It's not the most problematic strategical blow ever, but it's relevant enough that getting rid of it is a general principle of continental powers. Period.
Calling Dunkerque "only a small harbour" makes me thing you didn't put half the tought to your OP you might have wanted to : it was an important trade and military center since the late XVIIth century, one of the main harbours of French navy since Louis XIV, to not mention its role in maritime trade.
And frankly, if it was so unimportant, nobody would have mattered with annexing it in first place ITTL.
and I did not say all French Flanders, but only the northern part called Westhoek.
Westhoek is a relatively recent concept that didn't existed as such in early XIXth. You could as well have asked an Sardinian annexation of Arpitania.
You'd have no real reason, at this point, to dissociate Flandre flaminguante to the rest of French Flanders.
It would perhaps make France weaker, but they also gave up a region around Philllipville and Saarlouis.
I'm not sure you read the posts you answer there : I specifically mentionned the loss of Sarre between the treaties of 1814 and 1815 to point that, in the case of a third treaty of Paris due to revolt or any other happenance, you might see France loosing territories compared to 1815.
Now, in the context of IOTL Congress of Vienna, I wouldn't see it happening for all aformentioned reasons.
You would not need to have the strongest possible France, after the Napoleonic wars.
You'd be kind to stop the strawman argument there.
If you took the time to read my posts, you'd see that I pointed two things : one was that Great Powers wanted to return as much as possible to pre-revolutionary status-quo which implies not crushing France (the only Great Power that really argued a bit of this was Prussia, but it was the most weak of the five), and second that the point was to preserve the geopolitical status on the continent which did implied as well to not screw French positions.
Either disagree or agree but don't strawman it.