1992 Russia offered Kaliningrad to both Poland and Lithuania, but neither country was interested in a region with seriously underdeveloped economy and inhabited by Russians who wouldn’t feel either Polish or Lithuanian, nor would they speak the language.

> And if you look at the borders, it's pretty clear they were designed as a lock

Assuming you speak about Suwalki Corridor, those borders reflect pre-war borders of local counties. No mastermind planning to make it hard for Russia to make it from Belarus to Kaliningrad. In fact it was Russia who drove those lines in first place after WW2.

Only „problem” Russia would have to solve in eventual aggression is getting an army into Belarus. But they can just target the Baltics instead and strike from their own territory.