I have to say that I am rather surprised that Cologne ended up being part of France post war. Cologne is not part of the Saarland Geon and is atcually very far off the French border and closer to Belgium or the Netherlands than to France.
It is far more likely that Cologne would end up being part of a Greater Netherlands, as it was included in Area A of the Bakker Schut plan:
http://en.wikipedia.org/wiki/Dutch_annexation_of_German_territory_after_World_War_II
The most France can hope to get away with in my opinion, is the area bounded by the Rhine and Moselle rivers, which roughly correspond to the Palatinate. A French annexation of said area was proposed as part of the Morgenthau plan.
The area would be integrated into France as two new départements, which will likely benefit from a special regime very similar to the one existing in Alsace-Moselle:
http://en.wikipedia.org/wiki/Local_law_in_Alsace-Moselle
France does appear to be a very centralised and very Parisian centric state from the outside. This is undoubtedly very true in a lot of respects, but French local authorities do wield a lot of power and influence at the lower levels. Corsica also benefits to a degree from a status similar to Alsace-Lorraine, albeit one that is far less wide ranging.
The special status in place in Alsace-Moselle, goes as far as original law texts written in German having primacy upon French translations. It was even permitted for local politicians to run political campaigns in German alongside French for practical reasons.
There will undeniably be a population transfer of sorts, akin to the one that took place in Alsace in 1919 to weed out recent migrants and collaborators. There will also be moves to "Frenchify" the area in a lot of respects. But from the fifties onwards, the status quo will gain favour and traction among the local population. Sure there may be bitter divisions between pro French and pro German sides, but considering that the place will become one of the main industrial centres of France, prosperity may create some vested interests locally that are happy with the status quo.
Deindustrialisation and the decline of coal mining further down the line, could led to a drastic reversal of fortunes though and if the area feels "abandonned" by France then all bets could be off. But considering the special set of circumstance that led France to embrace nuclear power like it did OTL, the same may not happen TTL and the French electricity grid might run on Saar coal for a very long time.
Regarding European economic prosperity with a divided Germany.
I think that it is very simplistic to assume that a divided Germany automatically means a poorer Europe.
There is nothing that is made in Germany that cannot be manufactured in either France, Britain or the Low Countries. The removal of German competition will in fact give a shot in the arm of non German industries. ICI of Britain will take the place of BASF. Renault or Austin, of Volkswagen. France welcomed a lot of German technicians and engineers to kick start its jet engine manufacturing industry. The same will happen here on a much larger scale, with Britain and Scandinavia joining in too!
Now sure, a poorer Germany means a poorer Europe overall, but it won't prevent Western Europe from undergoing its own economic miracle. The centre of gravity of economic Europe may even in fact move west TTL, potentially never to return to the Rhine valley and Germany until the 2030s or even later.
The Europe of TTL will be led by Paris and London. With the potential for a lot of strategic cooperation between the two powers depending on how things go. I think for example that it is a near given that in the absence of German industry, TTL equivalent of Airbus will be purely Anglo-French.
) and the Germans joined to complete their application for readmission to the human race (even more so here...afraid I fall in the camp that doesn't see the French giving terribly much back either).