AHC/WI Grand Duchy of Alsace in PU under Louis XVIII after Napoleonic Wars

What if Alsace is split from France in 1814/15, but as a personal union under Louis XVIII and member of the German Confederation? (As for instance Luxembourg, Hannover and Holstein & Lauenburg)

So last night I read this old thread about Alsace-Lorraine in 1815 (https://www.alternatehistory.com/discussion/showthread.php?t=215990) and found the discussion about who would be its ruler and if it would become part of Germany or France really interesting.

So here are some points/questions/challenges to which I will post my own opinions below:
1) How is this achievable? Does this step have to be included in the first treaty of Paris (1814)? Or could this be decided in Vienna, since it's just a transfer of land between realms not monarchs (Congress goal of restoration)? Alternatively, is it more likely as a reaction to Napoleon's 100 days?

Well, in my mind such a scenario would have to be first brought up during the campaign in 1813/14. There were some voices in Prussia who wanted to take Alsace away from France, and others who just wanted to punish it more harshly. Once the idea has got some backing it could resurface during the Congress of Vienna. Here, I guess Talleyrand would fight it tooth and nail, but once Napoleon returns from Elba the French position is undermined and it could become part of the second treaty of Paris (1815).

2) As a member of the German Confederation would there be confederation troops stationed in Alsatian forts? Alsace’s incorporation into the French customs system was only finished 25 years prior (1789), would it remain this way? What would be its exact borders?

These are all very intertwined so let me begin with the simplest.
I think Alsace would remain part of the French customs system, especially because there is no German alternative in sight at this point in time.
To balance for this French connection there will be designated fortresses garrisoned by confederate troops (Belfort(see below), Muhlhausen, Huningen, LA Petite-Pierre, Neuf Brisach, …) to guard (Alsace and “Germany”) against France.
In the south Belfort either remains part of Alsace or its fortress is demolished. In the first case it will be a garrisoned by german troops and Muhlhausen and Huningen demolished as IOTL. Otherwise with Belfort gone the German confederation will probably keep one of those (my guess would be Muhlhausen).
In the north the border could end up meeting the OTL Franco-German border pretty much anywhere between just west of Sarreguemines or Wissembourg.

3)What are the most obvious consequences both within France and the German Confederation and in Europe in general?

Assuming all this (French PU with Alsace within German Confederation) comes to pass with minimum or next to no changes (that could have their own butterflies) I can think of three thing of the top of my head.
i) French involvement in the German Confederation similar to British, Danish and Dutch involvement, especially (Butterflies!) as 1848 comes knocking.
ii) The July Revolution, I doubt Austria and/or Prussia will go to war over succession in Alsace but maybe require concessions that the Karlsbader Beschlüsse (Press censorship and other reactionary measures) remain in effect. (I am unsure how liberal the July Monarchy truly was)
iii) In Strasbourg Napoleon III. first tried to seize power.
 
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