The first treaty of Paris had already been quite generous with France (borders of 1792) and I doubt that France might seriously ask for Antwerp and the former Austrian Netherlands. The resources of Flanders are not yet so obvious as they would become after 1830, but I cannot see any British government accepting a French control of Antwerp. Not only this would negate the strategy of French containment but it would also mean that the Netherlands would get no compensation for the loss of the Cape and Ceylon. Even when the danger of a Napoleonic resurgence was out of the picture (Belgian insurrection of 1830) there was never a chance that France would be allowed to take Flanders, notwithstanding all the attempts made by Talleyrand.
The strategy of French containment was also the reason for which Prussia (which had already a number of exclaves in the area) was awarded the balance of the Rheinland (again 1st treaty of Paris). Prussia was not exactly elated since there was no territorial contiguity between Brandenburg and the Rheinland, and they would have by far preferred Saxony. The compromise reached by the powers in February 1815 was to grant Prussia 40% more or less of Saxony, but to keep Saxony independent (Russia and Prussia had different ideas: Prussia was supposed to gain all of Saxony, while Russia would be compensated with Prussian territories in Poland, and would have been allowed to include all the Polish lands in the "kingdom of Poland".
As far as the Austrians were concerned, they did not want Saxony to disappear, since it would have resulted in an excessive strengthening of Prussia, and did not want a large kingdom of Poland either, which might have ambitions on Kracow and Galicia. The Austrians did also want Murat ousted from Naples, and they got this too.
The British had no interest at all in enlarging their continental possessions: Hanover was already seen as a problem child, since it would have been very hard to defend. Adding Westphalia or whatever else in western Germany would have not made things better, rather the contrary. The British were quite happy in gorging up in the colonies, and keeping choice bits like Malta, the Ionian islands and Heligoland.
Changing the succession law in Hanover was theoretically possible (it had been done 70 years earlier in Austria with Marie Therese), but it was never considered seriously and I doubt very much it would have been acceptable to the British parliament.
The compromise reached in Vienna in February 1815 was quite a reasonable one: everyone got something, even if not all they would have wanted. IMHO this is the strongest reason for which a war pitting Russia and Prussia against UK, Austria and France. The second strongest reason was that all the powers wanted an end to major wars, after almost 20 years of uninterrupted warfare.
I tend to agree with the Belgium issue.
Do you think the British Parliament would have a say in the matter of adding Westphalia to Hanover?
Remember, the British Parliament had no say over the governance of Hanover. It was not a colony of Britain but held by George III (and administered by this point by his sons) and was as separate nation. If the monarchy was fine to take it, I can't seem Parliament doing anything about it. They wouldn't launch a coup d'état against the King or invade. They would just openly state that no British government would do anything to help Hanover should it get itself into trouble.
I know Britain often thought that they were obligated to help defend the King's "Other Realm" in the past (7 Years War) but they can't refuse to allow the King to take this additional land to Hanover if he wanted to.
The biggest issue is if Britain was unhappy with other facets of the Treaty (the elimination of Saxony) while the King was happy with it. It would be embarrassing if the King signed it on behalf of Hanover while Britain refused.