This is completely wrong, the entire Congress was surrounded by the topic of legitimacy. Since the abdication of Napoleon, the restoration of legitimacy was pushed via the restoration of the Bourbons -- even if they were not all that popular. The crisis over Saxony stemmed from the attempt by Prussia to deprive a monarch of their legitimate holdings. The topic if Naples regarding the legitimacy of Murat's rule. The accession treaties with German states such as Bavaria (Treaty of Ried), Wurrtemburg (Treaty of Fulda), etc. that maintained much of the 1813 status quo legitimized parts of the Napoleonic order. Legitimacy is everything, which is why unsanctioned land grabs are frowned upon. France itself proclaimed itself as the protector of legitimacy during the Congress to enable the Bourbon regime. The so called "many German states" that I believe you are talking about were legitimately deposed via the Reichsdeputationshauptschluss. There's certainly power-politics at play with territorial transfers and seizures at the Congress, but there was certainly a willingness to conduct it in good faith.
More Britain than anyone else.
The reason Russia returned Tarnopol was because Alexander falsely presumed Francis would be more present during the negotiations and appease Russia's desires for Poland. This didn't happen and Francis kept an off-hands approach to the Congress & left most things to Metternich sans a handful of instructions. Tarnopol was not that important so I don't see it influencing much of anything regardless if Russia kept it, especially in regards to the Austrian Netherlands.
Then what exactly is the point of this thread? Let Austria keep it...only to have them give it away just as otl. The logical conclusion here is that Austria is not getting back the Lowlands unless you have a completely different POD further back that alters the Revolutionary/Napoleonic Wars as we know it.