What is Lithuanian Federation???
There is no Lithuanian federation.
I assume you know what a federation is, so the easiest explanation I can give you is to have a brief study of the Swiss Federation and compare like for like.
Effectively, Kaliningrad would become a 'canton'.
This is also the proposed political system for Ireland when it will be united with the North of Ireland. It is successful in giving regions a large amount of autonomy, while still maintaining unity.
Of course, language and cultural barriers will always be a problem, as we have seen Belgium has been rocked to the core in recent years.
This is the primary reason why it would not be returned to Prussia/Germany or joined with Poland - i.e. you can still speak Russian in most Lithuanian cities with the exception of Kaunas.
Then of course, the issues with geography and shared past. i.e. the Russian Diaspora.
Ultimately of course, this is mental chewing gum at the moment.
Geo-politics would exempt any such scenario until there is normalisation of relations between the Baltic and Russia...................which could be some time away.
I think the biggest issue at the moment in Russia's eyes, is a potentially impending civil war in the Ukraine, between the Western Nationalists and the South Eastern Pro-Russian communities.