The whataboutism is strong here.
FYI, I agree that Catalonia, Scotland and Northern Ireland should be independent but this discussion isn’t about them.
Secondly, comparing the life that Ukrainians will have under Russian annexation to a Scottish person in the UK is so ludicrous.
Your argument about post-WWI carving can be made about several nation states in Europe.
If this was 1939, you would be justifying Anschluss and the capturing of the Sudetenland.
I know this ain't about Scotland, Catalonia or Northern Ireland but those analogies are historically flawed. Apart from Ireland, the rest of the above countries were not subjected to cultural ethnic cleansing historically speaking. Scotland was never wholly Gaelic speaking as Lowland Scotland was historically English speaking (Scots) due to the Germanic settlement of Southern Scotland. English gradually became the dominant language of the Scottish aristocracy even before the union of England, Scotland and Ireland when the Scottish James IV/James I became the sole sovereign. Ironically, it was this Scottish King that started to promote Scots at the expense of Gaelic in the Highlands.
Catalonian Nationalism is a 19th century creation by some intellectual elites as there was no national consciousness in centuries past, the same obviously applies to many other European nationalist movements. Their pre-19th century historical grievance against the Spanish State mainly stems from the fact that they were sidelined after Catalonia and others such as Valencia backed the wrong claimant (Hapsburg) to the Spanish throne during the War of Succession, and suffered the loss of privileges as a result. They never ever sought a historical divorce from the Spanish Crown that came into being after the Union of Aragon and Castile.
As for Ukraine, they did belong to the same historical ethnic group as Russians and Belarussians. However, centuries of a separate existence and the development of a distinct language caused them to organically become distinct from the main body of Rus. To deny them self-determination on the basis that they and the Russians have the same ethnic origin in the Kievan Rus is akin to asking the Netherlands to be dissolved and united with Germany because all Western Germanic speakers are Deutsch/Duits historically speaking.